In this article, we shall study structural units of nucleic acid called nucleotides.

In 1869, Friedrich Miescher separated cellular substance from the nuclei of pus cell and called it ‘Nuclein’. Due to acidic nature, the substance is further called as nucleic acid. There are two types of nucleic acids a) Deoxyribonucleic acid (DNA) found primarily in the nucleus of cells and b) Ribonucleic acid (RNA) found mainly in the cytoplasm of living cells.

Chemical Components of Nucleic Acids:

Nucleotides:

Nucleotides are the structural units of nucleic acids. Each nucleotide has three components

Sugars:

The five-carbon sugar (pentose) in nucleic acids is ribose or a ribose derivative. It has a pentagonal ring structure. In RNA the sugar is ribose, in DNA it is 2-deoxyribose.

The only difference between these two sugars is found at the 2-carbon of the ribose ring. Ribose has a hydroxyl group (-OH) bound to this carbon, while deoxyribose has a hydrogen atom (“deoxy” means no oxygen).

Phosphate Group:

The second component of a nucleotide is derived from phosphoric acid (H₃PO₄).

Phosphoric acid contains three hydroxyl groups attached to phosphorous.

Phosphoric acid             Phosphate group

From these three OH groups, two are responsible for strand formation.

Nitrogen or Organic Bases:

The organic bases found in nucleic acids are derivatives of pyrimidine or purine.

Pyrimidine is a six-membered heterocyclic ring. A heterocyclic ring is a ring compound containing atoms that are not all identical. Purine is a fused ring compound containing a six-membered ring connected to a five-membered ring.

Pyrimidines:

There is only one ring which is hexagonal and heterocyclic. The ring consists of four carbons and three nitrogens with an alternate single and double bond. Numbering is done clockwise starting from nitrogen. Nitrogen atoms are present at the first and third positions. Rest positions are occupied by carbon. Such a ring is called a pyrimidine ring.

The three pyrimidine derivatives found in nucleic acids are cytosine (C), thymine (T), and uracil (U).

Cytosine = 2-oxy-4-amino pyrimidine

Thymine = 2,4-dioxy-5-methyl pyrimidine

Uracil = 2,4-dioxy pyrimidine

Characteristics of Pyrimidines:

• They are single ring compounds.
• They are formed by a pyrimidine ring.
• There are 4 carbons and 2 nitrogens in the ring.
• Nitrogen atoms are present at the first and the third position.
• Oxygen is attached to second carbon by a double bond.
• A glycosidic bond is formed between nitrogen at the first position in pyrimidine and carbon at the first position in pentose sugar.

Purines:

There are two rings (dicyclic) in this nitrogen compound. There are nine atoms in the molecule of which 4 are nitrogen and 5 are carbon atoms. There are 6 atoms in the first ring called pyrimidine ring and 5 atoms in the second ring called imidazole ring. Atoms are numbered anticlockwise in pyrimidine ring and clockwise in the imidazole ring.  The imidazole ring.is fused with pyrimidine ring at the 4th and 5th position so that the two rings share carbon atom at 4th and 5th position. The nitrogen is present at first, third, seventh and ninth position in the ring.

The two purine derivatives found in nucleic acids are adenine (A) and guanine (G).

Characteristics of Purines:

• They are double ring compounds.
• They are formed by pyrimidine and imidazole ring.
• There are 5 carbons and 4 nitrogens in the ring.
• Nitrogen atoms are present at the first, third, seventh and ninth position.
• No oxygen is attached to the second carbon.
• A glycosidic bond is formed between nitrogen at the ninth position in pyrimidine and carbon at the first position in pentose sugar.

Note:

• Adenine, guanine, and cytosine are found in both DNA and RNA. Thymine is found only in DNA, while uracil is found only in RNA.
• Thymine and uracil are often used to differentiate DNA from RNA.

Nucleosides:

When ribose or 2-deoxyribose is combined with a purine or pyrimidine base, then the combination is called nucleoside. A nucleoside is basically a nucleotide that is missing the phosphate portion.

Thus Nucleoside = Sugar + Nitrogen Base

In a nucleoside, the pentose sugar and base are joined by an N-glycosidic bond formed between semialdehyde -OH group of monosaccharide at 1 and H of the pyrimidine base at N-1 or the purine base at the 9th nitrogen atom of the ring

New Naming System for Nucleosides:

Nucleotides:

The nucleotides are named according to their nitrogenous base. For e.g. a nucleotide containing thymine is called thymine nucleotide.

Thus Nucleotide = Pentose Sugar + Nitrogen Base + Phosphate Group

or Nucleotide = Nucleoside + Phosphate Group

New Naming System for Nucleotides:

Linking of Nucleotides in Polynucleotides:

A polynucleotide chain is formed by connecting several nucleotides in succession. Several thousand nucleotides are linked together by 3′-5′ phosphodiester bond in which the phosphate group carried in 5th carbon atom of pentose in one nucleotide is linked to 3′ hydroxyl group of 3′ carbon of the pentose of the next nucleotide. These bonds provide considerable stiffness to polynucleotide chain.

The bond is called phosphodiester bond because one molecule of phosphoric acid joins with sugar molecules of two nucleotides through an ester linkage.

Joining two nucleotides is called dinucleotide, joining three nucleotides is called trinucleotide and so on. A chain up to joining of twenty nucleotides is called oligonucleotide. If there is joining of more than twenty nucleotides it is called polynucleotide.

RNA is a polynucleotide that, upon hydrolysis, yields D-ribose, phosphoric acid, and the four bases adenine, guanine, cytosine, and uracil.

DNA is a polynucleotide that yields D-2′-deoxyribose, phosphoric acid, and the four bases adenine, guanine, cytosine, and thymine.

The Directionality of Polynucleotide Chain:

Adjacent nucleotides in a single strand of the polynucleotide are joined by a phosphodiester bond between their 3′ and 5′ carbons. This means that the respective 5′ and 3′ carbons are exposed at either end of the polynucleotide, which are therefore called the  5′-P end and the 3′-OH end. These are also called the phosphoryl (5′-P terminus) and hydroxyl (3′-OH terminus) ends, respectively, because of the chemical groups typically found at those ends.

Science > Biology > Gene its Nature, Expression and Regulation > Nucleotides

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DNA as Genetic Material

Griffith Experiment:

Background:

Meischer isolated nuclein from nuclei of WBCs in 1869. Walter Sutton, Thomas Hunt Morgan established that the hereditary material lies in the nucleus in chromosomes. Chromosomes are formed of proteins and nucleic acid, DNA. For many years proteins were assumed to be the carrier of hereditary information due to their structural and functional diversity. By 1926 the mechanism for genetic inheritance had reached the molecular level. But exactly which molecule is responsible for heredity was not confirmed.

Bacterium Streptococcus pneumoniae occurs in two strains:

• Smooth Virulent Strain (S-III): The smooth virulent strain of Streptococcus pneumoniae is enclosed in polysaccharide capsule. Due to the presence of the capsule their colonies are smooth and shiny. Hence they are called smooth strain (S). This capsule protects them by preventing them engulfed by WBCs. As they are not destroyed by WBCs, they cause pneumonia in mice.
• Rough Avirulent Strain (R-II): The rough avirulent strain of Streptococcus pneumoniae lacks polysaccharide capsule and hence are destroyed by WBCs. Due to the absence of the capsule, their colonies have an irregular appearance. Hence they are called rough strain (R). As they are destroyed by WBCs they do not produce symptoms of pneumonia in mice.

Experiment:

In 1928 Frederick Griffith, in a series of experiments with Diplococcus pneumoniae (bacterium responsible for pneumonia), witnessed a miraculous transformation in the bacteria. During the course of his experiment, the bacteria (living organism) had changed in physical form.

The pneumococcus bacterium occurs naturally in two forms with distinctively different characteristics. The virulent or pathogenic (S-strain) form has a smooth polysaccharide capsule that is essential for infection. The nonvirulent or nonpathogenic (R-strain) lacks the polysaccharide capsule, giving it a rough appearance.

Step – 1: S-type of the pneumococcus bacteria were injected into healthy mice. The mice were infected and died from pneumonic infection within a few days,

Step – 2: R-type of the pneumococcus bacteria were injected into healthy mice. The mice were not infected and continue to live.

Step – 3: Heat Killed S-type of the pneumococcus bacteria were injected into healthy mice. The mice were not infected and continue to live.

Step – 4: A mixture of heat-killed S-type and live R-type pneumococcus bacteria were injected into healthy mice. It produced lethal results. The mice died. On observation, Griffith discovered a mixture of R-Type and living forms of the S-type bacteria in the infected dead mice.

Conclusions:

Griffith hypothesized that something has transformed the non-lethal R-type avirulent bacteria into lethal S – Type virulent bacteria. The heat-killed S-strain bacteria should be responsible for it. This transformation is called Griffith effect or bacterial transformation.

Some “transforming principle”, enabled the R-strain to synthesize a smooth polysaccharide coat and become virulent. He further observed that the “transforming principle” was transferred to the next generation. Thus “transforming principle” should be genetic material. Further, it was proved that the “transforming principle” referred to by Griffith is DNA.

Avery, Macleod and McCarty Experiment:

In 1944  Oswald Avery, Collin Macleod and Maclyn McCarty performed the same experiment as that by Griffith but their aim was definite to locate the factor responsible for a transformation of non-lethal R-type bacteria into lethal S – Type bacteria. They used a test tube assay instead of mice.

They purified DNA, RNA, proteins and other materials from heat-killed S – type bacteria using corresponding dissolving enzymes. Then they mixed purified content with R – type to see which one could transform living R – type into S – type.

Only those mixed with DNA were transformed into S – type bacteria. When DNA was treated with Deoxyribonuclease, the DNA was digested and dissolved, there was no transformation of R-type bacteria into S – Type bacteria. This confirmed that  “transforming principle” is DNA. But scientist community at that time was not convinced.

Hershey – Chase Experiment:

Alfred Hershey and Martha Chase (1952) experimentally proved that DNA is the only genetic material. They worked with viruses that infect bacteria called bacteriophages (T2-phages).

The bacteriophage attaches by its tail to the bacteria and its genetic material then enters the bacterial cell and protein coat is left outside. The bacterial cell treats the viral genetic material as if it was its own and subsequently produces more virus particles. A large number of phage-DNA molecules are formed. Each of these DNA molecules develops its own protein coat forming daughter phage particles.

Hershey and Chase performed an experiment to discover whether it was protein or DNA from the viruses that entered the bacteria.

Step – 1: 

They used the fact that DNA contains phosphorus but not sulphur, while protein contains sulphur but not phosphorous. They grew some viruses on a medium that contained radioactive phosphorus (³²P) and some others on the medium that contained radioactive sulphur (³⁵S).

Observations: Viruses grown in the presence of radioactive phosphorus contained radioactive DNA but not radioactive protein. Similarly, viruses grown on radioactive sulphur contained radioactive protein but not radioactive DNA.

Step – 2:

Radioactive phages were allowed to attach to E. coli bacteria. As the infection proceeded, the viral coats were removed from the bacteria by agitating them in a blender and the virus particles were separated from the bacteria by spinning them in a centrifuge.

Observations:

Bacteria which was infected with viruses that had radioactive DNA were radioactive, indicating that DNA was the material that passed from the virus to the bacteria. The phages grown in radioactive phosphorous passed their radioactivity to the daughter phage particles through DNA.

Bacteria that were infected with viruses that had radioactive proteins were not radioactive. The phages grown in radioactive sulphur did not pass their radioactivity to the daughter phage particles through proteins. This indicates that proteins did not enter the bacteria from the viruses.

Conclusion:

Therefore DNA is the genetic material that is passed from virus to bacteria.

RNA as Genetic Material

Frankel-Conrat and Singer Experiment:

H. Frankel-Conrat and B. Singer (1957) performed an experiment with tobacco mosaic virus (TMV) and demonstrated that in some cases RNA acts as a genetic material.

Tobacco mosaic virus (TMV) does not contain any DNA. It consists of RNA surrounded by a hollow cylinder of protein subunits. They found that the virus could be broken into component parts and they could again be reassembled or reconstituted to form a functional virus.

Viruses with the single-stranded genome (RNA) use a single strand as a template and synthesize a complementary single strand of DNA. This complementary single-strand DNA, in turn, synthesize its complementary strand and forms a double-stranded DNA.

Techniques were first developed for separating TMV particles into RNA and proteins. Later by using RNA and proteins separately in tests for infectivity, it could be shown that RNA alone was able to cause infection. Such property was not found in the protein fraction.

When the cell debris (protein coat) of the virus was introduced into tobacco leaf, the leaf remained healthy. When the cell filtrate (nucleic acid) was injected into tobacco leaf, it was infected with the virus and died. This shows that the RNA is causing the infection and not the protein.

The progeny viruses produced were always found to be phenotypically and genotypically identical to the parent strain from which the RNA had been obtained.

In one experiment, two viruses used were tobacco mosaic virus (TMV) and Holmes rib-grass virus (HRV). Reciprocal hybrid using RNA of one strain and protein of the other strain is obtained. It was found that when these hybrids were used for infection, the progeny had proteins which corresponded to the virus from which RNA of the infecting virus particles was derived.

Properties of DNA in Genetic Material:

• DNA has the ability to store hereditary information in coded form.
• DNA is present in all the cells of the organism.
• DNA shows diversity corresponding to the varieties existing in the organisms.
• DNA has the capacity to replicate itself to produce a carbon copy that could be transferred to daughter cells (successive generations).
• DNA is able to express itself through specific biological molecules like proteins and enzymes.
• DNA has physical and chemical stability so that the stored information is not lost.
• DNA (genes) is capable of differential expression so that the various parts of an organism may acquire specific form, structure and functions in-spite of having the same genetic material.
• DNA (genes) undergoes gradual mutations and recombinations so that the new characters appear in the organism to produce diversity.

Comparision Between DNA and RNA as Genetic Material:

• DNA is the genetic material in most organisms except in plant viruses and some animal viruses where RNA acts as genetic material.
• Both have a stable structure and yet capable of undergoing mutations (slow changes).
• Both are capable of transcription and translation.
• As both DNA and RNA follow base pair-rule and hence exhibit complementarity. Both of them have the ability to direct their duplication.
• DNA is very stable while RNA is more reactive (less stable).
• RNA mutates faster than DNA
• RNA can code for the synthesis of protein directly while DNA depends on RNA to transfer the message of protein synthesis from the nucleus into the cytoplasm.
• From the above points, we can conclude that DNA is more stable. Hence are more suited for storing genetic information.

Science > Biology > Gene its Nature, Expression and Regulation > Genetic Material

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In the last article, we have studied the meaning of the term gene. In this article, we shall study types of genes.

Based on the function and activity, the genes are classified as follows.

Housekeeping Genes or Constitutive Genes:

Housekeeping genes are involved in basic cell maintenance and, therefore, are expected to maintain constant expression levels in all cells and conditions. They are functional in all types of body cells of a multicellular organism and all the time. They are required for basic cellular activity. They are not regulated.

Example: Genes associated with glycolysis are active in all types of cells and all the time throughout life.

housekeeping genes are instrumental for calibration in many biotechnological applications and genomic studies. Advances in our ability to measure RNA expression have resulted in a gradual increase in the number of identified housekeeping genes.

Luxury Genes or Noncontitutive Genes:

These genes are not always expressing themselves in a cell. They remain inactive for most of the time in the lifespan of an individual and is expressed in certain cells or at a certain time only when their products are needed. These are called luxury genes or specialist genes.

Humans comprise approximately 200 different types of cells, such as skin cells, liver cells, and nerve cells. Each cell varies in both the structure and the function because different sets of genes are expressed in each of them. For example, the serum albumin gene is expressed only in hepatocytes (liver cells), while the insulin gene is expressed only in pancreatic beta cells. They are switched on or off according to the requirement of cellular activities.

Example: the gene for nitrate reductase in plants, lactose system in Escherichia coli. There are some genes in the human body which are present in all the body cells but some are functional in kidney cells, some in liver cells and some in intestine or stomach. They are associated with adaptive enzyme synthesis.

Luxury genes are of further classified as inducible and repressible. The genes are switched on in response to the presence of a chemical substance or inducer which is required for the functioning of the product of gene activity are called inducible genes, e.g., nitrate for nitrate reductase. The genes which continue to express themselves till a chemical (often an end product) inhibits or represses their activity are called repressible genes.

Structural Genes (Cistrons):

These genes code for chemical substances which contribute to the morphological or functional trait of the cell. These are called cistrons. They are continuous in prokaryotes and split into introns and exons in eukaryotes. They are further classified as

• Polypeptide-coding Genes: These genes code for mRNAs which in turn code for polypeptides. The polypeptide produced may act as a component of an organelle (as actin of muscle fibre); an enzyme (as DNA polymerase); a transport protein (as haemoglobin); a hormone (as insulin); a receptor or carrier protein of cell membrane; an antibody, an antigen.
• Polyprotein-coding Genes: These genes code for more than one polypeptide per gene.
• RNA-coding Genes: These genes code for rRNAs and tRNAs.

Regulator Genes:

These genes code for repressor proteins for regulating the transcription of cistrons.

Operator Genes:

An operator gene acts as a switch to turn on or off the transcription of a structural gene as and when required by the cell.

Promoter Genes:

These genes are DNA sequences (sites) for the binding of RNA polymerase for the transcription of RNAs by the structural genes.

Terminator Genes:

These genes are DNA regions (lying t end of message) where RNA polymerase activity stops to suspend transcription of structural genes.

Uninterrupted Genes or Continuous Genes Or Collinear Genes:

In prokaryotes, the sequence of nucleotides in the gene corresponds exactly with the sequence of amino acids in the protein. Such nucleotide sequence codes for a particular single polypeptide chain.  Each gene is a continuous stretch of DNA whose length is related to the size of protein to be synthesized. Thus these genes and proteins are collinear.

Interrupted Genes or Discontinuous Genes or Split Genes:

Generally, a gene has a continuous sequence of nucleotides. However, it was observed that the sequence of nucleotides was not continuous in the case of some genes, the sequences of nucleotides were interrupted by intervening sequences. Such genes with the interrupted sequence of nucleotides are called split genes or interrupted genes. Thus, split genes have two types of sequences, viz., normal sequences and interrupted sequences

The Concept of Exons and Introns:

The coding units containing biological information are called exons. and intervening non-coding DNA segments are called introns. Introns are present in the genes of eukaryotes, viruses, and archaebacteria. Interrupted genes produce the primary transcript RNA. It acts as a precursor as it is a faithful copy of the interrupted gene.

The functional RNA is formed by the removal of introns and rejoining exons. This process is known as RNA splicing.

Overlapping Genes or Alternate Genes:

A few genes in certain bacteria and animal viruses code for two different polypeptides (more than one protein). These are called overlapping genes. In this case, the specific sequence is shared between two non-homologous proteins. In these genes, the first and second half of the gene codes fora specific protein that represents the first or second half of the protein, specified by the full gene.

Alternative Genes:

The concept of alternative genes was given by Gilbert and is known as Gilbert hypothesis. They are formed when exons from different discontinuous genes get connected forming several new combinations. These genes produce proteins in which one part is common while another part is different.

Jumping Genes or Transposons: 

They are segments of DNA that can jump or move from one place in the genome to another. Transposons were first discovered by Nobel prize winner Mc Clintock (1951) in the case of Maize when she found that a segment of DNA can move from one position to another in the genome of the cell. Recently they have been described in snapdragon, Drosophila, mice, and bacteria.

Transposons possess repetitive DNA, either similar or inverted, at their ends. The two major events took place during transposition. There is a duplication of the target sequence in the recipient DNA molecule and the insertion of transposons between the repeated target sequences.

Gene Families and Pseudogenes:

They are genes which have homology to functional genes but are unable to produce functional products due to intervening nonsense codons, insertions, deletions, and inactivation of promoter regions, Pseudogenes are genomic DNA sequences similar to normal genes but non-functional; they are regarded as defunct relatives of functional genes.

Most of the prokaryotic genes are represented only once in the genome. But many eukaryotic genes are presented in multiple copies. These multiple copies of genes are called gene families or pseudogenes. They may be clustered in the same region of DNA or dispersed to different chromosomes.

e.g., several of snRNA genes.

Single Copy Genes:

The genes are present in single copies (occasionally 2-3 times). They form 60-70% of the functional genes. Duplications, mutations and exon reshuffling between two genes form new genes.

Processed Genes:

They are eukaryotic genes which lack introns. Processed genes are generally nonfunctional as they lack promoters.

Multi-genes (Multiple Gene Family):

It is a group of similar or nearly similar genes for meeting the requirement of time and tissue-specific products.

Science > Biology > Gene its Nature, Expression and Regulation > Types of Genes

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In this article, we shall the essential characters of genetic material, the meaning of the term gene, its characteristics, and its functions.

Essential Features of Genetic Material:

• It should have the ability to store hereditary information in coded form.
• It should be present in all the cells of the organism.
• It should show diversity corresponding to the varieties existing in the organisms.
• It should have the capacity to replicate itself to produce a carbon copy that could be transferred to daughter cells (successive generations).
• It should able to express itself through specific biological molecules like proteins and enzymes.
• It should have physical and chemical stability so that the stored information is not lost.
• It should be capable of differential expression so that the various parts of an organism may acquire specific form, structure and functions in-spite of having the same genetic material.
• It should undergo gradual mutations and recombinations so that the new characters appear in the organism to produce diversity. Thus The genetic material should be able to generate its own kind and also new kinds of molecules.

Gene:

A gene may be defined as a segment of DNA which is responsible for inheritance and expression of a particular character. A gene is a segment of DNA that provides instructions for the synthesis of a specific protein or a particular type of RNA.

Mendel was first to call genes as a unit of inheritance and called them factors. The term ‘gene’ was derived from the Greek word ‘Genesis’ which gives the meaning ‘to be born’ and was coined by a Danish Geneticist- Wilhelm Johannsen in 1909.

Characteristics of Genes:

• Genes are the functional unit of heredity, variation, mutation and evolution. Genes determine the physical as well as physiological characteristics of organisms. Genes are responsible for transferring these characters from parents to the offspring generation after generation.
• They are situated in chromosomes.
• Every gene occupies a fixed position in a chromosome. This position is called a locus.
• They are arranged in a single linear order in a chromosome as beads on a string.
• They express them by the synthesis of proteins and enzymes, which control cell metabolism. Thus they determine the physical and metabolic characteristics of the cell. Each gene synthesizes a particular protein which acts as an enzyme and brings about the appropriate change.
• They can produce a duplicate copy of themselves. The process is called replication.
• In a single gene they may occur in several different forms called alleles. Only those genes are known which have their alternative alleles. The alleles may be related as dominant or recessive but not always.
• Some alleles mutate more than once and have more than two alleles. These alleles are known as multiple alleles. Whatever may be the number of alleles in a multiple series only two of them are found in an individual because of the presence of two homologous chromosomes of each type.
• They may show a sudden change in expression from one form to another due to a change in composition. This sudden change is called mutation and the new allele is called a mutant.
• There is a large number of genes in organisms while the number of chromosomes is small. Hence several genes are located in each chromosome. In the human being, there are about 40,000 known genes located on 23 chromosomes.
• A gene is a segment of DNA which contain information for the synthesis of one enzyme or one polypeptide chain coded in the language of nitrogenous bases or the nucleotides.

Modern Concept of Gene:

Seymour Benzer in 1955 introduced the terms cistron, muton, and recon

Cistron (Unit of function):

• It is a segment of DNA having information of synthesis of particular protein or RNA.
• It is responsible for the expression of a trait.
• It can be several bp (base pairs) long.

Muton (Unit of mutation):

• It is a segment of DNA that can undergo mutation.
• It consists of few nucleotides (one to a few bp long).

Recon (Unit of recombination):

• It is a segment of DNA that participates in recombination through crossing over during meiosis.
• It consists of a few to many base pairs.

Operon: 

• It is a combination of an operator gene, a structural gene or sequence of structural genes which act together as a unit.

Replicon:

• It is the unit of replication

Functions of Genes:

• Genes are the functional unit of heredity, variation, mutation, and evolution. Genes determine the physical as well as physiological characteristics of organisms. Genes are responsible for transferring these characters from parents to the offspring generation after generation.
• Genes control the phenotypes of the offspring including both the structural and functional characters.
• Genes control reproduction through their replication.
• Genes undergo mutations and produce polymorphism and variations in the individuals of a population. These mutations are also associated with metabolic disorders and inborn errors of metabolism.
• Genes are associated with the aging process.
• Genes are responsible for producing cancer.
• Control genes regulate transcription of mRNA and thus regulate the amount of protein synthesized.
• They code for different types of RNAs other than mRNA like rRNA and tRNA.
• Genes are responsible for switching on and off specific genes as per the requirement of the organism.
• Genes control the functioning of luxary genes.
• They produce cellular differentiation during development.

Science > Biology > Gene its Nature, Expression and Regulation > Gene: The Concept, Characteristics, and Functions

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The first scientific explanation of inheritance was given by Mendel in 1866. He performed a series of experiments on garden pea in a scientific manner and proposed rules. which are called as Mendel’s Laws of Inheritance. His work is known as Mendelism. He laid down a foundation of Genetics hence he is called Father of genetics. In this article, we shall studyMendel’s laws of inheritance.

Mendel’s Law of Unit Characters or Unit Factors:

Statement:

Genetic characters are controlled by unit factors. These factors exist in pairs in organisms.

For example in each cell of a tall plant, there are two factors for tallness. Similarly each cell of a dwarf plant, there are two factors for dwarfness.

Mendel’s Law of Inheritance (Law of Dominance):

Statement:

When two unlike unit factors, responsible for a single character, are present in a single individual (F₁ hybrids) only one unit factor expresses itself. The character that appears in F₁ generation is called dominant and the one which is suppressed is called recessive. This law is also referred as Mendel’s first law of inheritance.

Explanation:

Let genotype of a hybrid offspring (F₁ generation) be (T,t) which is heterozygous. Where T is an allele for tallness and t is an allele for dwarfness, As T is the dominant allele and t is recessive allele, the phenotype of hybrid is a tall plant. Thus the allele for tallness expressed itself and has masked the expression for the allele for dwarfness.

This law can be explained using monohybrid cross experiment.

The law is significant and true but is not universally applicable. There are some cases where dominance is not complete or absent. Thus there are cases of incomplete dominance or co-dominance. This cases can be explained by studying cases of deviations from Mendelian inheritance. Hence the law of dominance is not universally applicable.

Mendel’s Law of Inheritance (Law of Segregation):

Statement:

Members of an allelic pair in hybrid remain together without mixing with each other and separate or segregate during gamete formation. This law is also referred as Mendel’s second law of inheritance. This law is universal.

Thus gametes receive only one of the two factors and are pure for a given trait. Hence this law is also known as the Law of Purity of Gametes.

All the sexually reproducing organisms are diploid (2n) i.e. with two sets of chromosomes and gametes are haploid (n) i.e. with one set of chromosomes.

The Significance of Law of Segregation:

This law introduced the concept of heredity factors as discrete, physical entities that they do not become blended or altered when present together in the same individual.

He disproved the blending theory by showing that although traits caused by recessive alleles disappear in the F1 generation, they reappear unchanged in F2 generation.

Limitations of Law of Segregation (Mendel’s Laws of Inheritance):

• This law is applicable to only diploid organisms which form haploid gametes during sexual reproduction.
• This is law is applicable to organisms having single gene pair containing two alleles one dominant over the other.
• This law is not applicable to alleles that are incompletely dominant or co-dominant.
• This law is not applicable to genes those collaborate or vary in their expression or in penetrance.
• This law is not applicable to genes which are pleiotropic or complementary.
• This law is not applicable to traits caused by many gene pairs.

Mendel’s Third Law of Inheritance (Law of Independent Assortment):

When two homozygous parents differing in two pairs of contrasting traits are crossed, the inheritance of one pair is independent of other. This law is also referred as Mendel’s third law of inheritance.

In other words, when a dihybrid (or polyhybrid) forms gametes, assortment (distribution) of alleles or different traits is independent of their original combinations in the parents. This law can be explained by help of dihybrid cross and dihybrid ratio. The appearances of new combination prove the law. The law is universally applicable.

It is immaterial whether both dominant characters enter the hybrid from the same or two different parents but the segregation and assortment remain the same. The appearances of new combinations prove the law. The law is universally applicable.

Importance of Mendel’s Laws:

• The concept of dominant and recessive factors is very important. This character is shown by many hereditary traits.
• It gives an idea of new combinations of traits which are very useful in developing a desirable trait in a progeny.
• This information is particularly used in the field of plant and animal breeding. Thus a new type of plants and animals can be produced by hybridization.

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Mendel performed his experiments with garden pea plant, which has traits or alleles having complete dominance and hence the laws of inheritance were proved. Other scientists performed their experiments on different plants and animals and found deviations to Mendelian ratios. Depending upon these experiments and observations, a different pattern of inheritance called gene interactions was discovered. This study is known as Post – Mendelian genetics or Neo-Mendelian genetics. In this article, we shall study the concept of polygenic inheritance.

Polygenic Inheritance or Quantitative Inheritance:

These characters are determined by two or more gene pairs and they have an additive or cumulative effect. These genes are called cumulative genes or polygenes or multiple factors. Polygenes are two or more different pairs of nonallelic genes, present on different loci, which influence a single phenotypic character and have an additive or cumulative effect. They are also called quantitative genes or cumulative genes or multiple factors.

A single phenotypic character governed by more than one pair of genes is called polygenic character or quantitative character. Polygenic characters or quantitative character show continuous variation. Galton (1883) predicted that in human population characters such as height, skin colour and intelligence show continuous variations in expression and not only two contrasting expressions.

In cumulative or polygenic inheritance each gene has a certain amount of effect. So more is the number of dominant genes, the greater is the expression of the character. It is generally believed that during evolution there was a duplication of chromosome or chromosome parts. This resulted in multiple copies of the same gene. Note that Mendel studied qualitative inheritance, where complete dominance is observed.

Polygenic Inheritance in Wheat Kernel Colour:

Swedish geneticist H. Nilsson-Ehle discovered polygenic inheritance. He crossed a red kernelled variety of wheat with white kernelled variety. In F₁ generation all plants have grains with intermediate colour between red and white. In F₂ generation five different phenotypic expressions (the darkest red, medium red, intermediate red, light red, white) appeared in the ratio 1:4:6:4:1. Nilson Ehle suggested that the kernel colour in wheat is controlled by two pairs of genes, Aa and Bb. Genes A and B determine the red colour. a and b which do not produce red colour pigment and their expression is a white colour of the kernel.

Polygenic Inheritance in Human Skin Colour:

The presence of melanin pigment is responsible for the colour of the skin in a human being. Each dominant gene is responsible for the synthesis of a fixed amount of melanin. The amount of melanin synthesized is directly proportional to the number of dominant genes.

The amount of melanin developing in persons is determined by three pars of genes A, B, C. These are present on three different loci and each dominant gene is responsible for the synthesis of a fixed amount of melanin. A genotype of a pure black parent in which melanin is produced is the highest is AABBCC, while that of pure white also called albino no melanin is formed is aabbcc.

Mulattoes i.e. F₁ offspring produce (2³ = 8) different types of gametes. Let us consider mulatto intermediate whose genotype is AaBbCc. By doing cross among two mulatto intermediate we get (2⁶ = 64) combinations in F₂ generation. But there only 7 phenotypes due to a cumulative effect of each dominant gene.

When we analyze all possible combinations and plot the probability graph by taking frequency distribution of colour, the number of dominant genes in various shades on the x-axis and the frequency of different shades onthe y-axis. In Polygenic inheritance often we get a bell-shaped curve as shown below.

This means that most people fall in the middle of the phenotypic range, such as skin colour, while very few people are at the extremes, such as pure white or pure dark. At one end of the curve will be individuals who are recessive for all the alleles (for example, aabbcc). They are rare; at the other end will be individuals who are dominant for all the alleles (for example, AABBCC) they are rare. In the middle of the curve will be individuals who have a combination of dominant and recessive alleles (for example, AaBbCc or AaBBcc). The graph also shows that the expression level of the phenotype is dependent on the number of contributive alleles and hence more quantitative.

Other examples are the height of human being, cob length of maize.

Comparative Study of Qualitative and Quantitative Inheritance:

Qualitative Inheritance:

• Qualitative characters are classical Mendelian traits which have two contrasting expressions and are controlled by a single pair of genes. e.g. tall and dwarf pea plants. A qualitative character can be expressed by a single pair of the gene. Hence the traits are called monogenic traits. The inheritance of monogenic traits (monogene) or qualitative characters is called qualitative or monogenic inheritance.
• A qualitative trait is expressed qualitatively, which means that the phenotype falls into different categories. These categories do not necessarily have a certain order.
• Qualitative inheritance was first studied by Mendel.

Characteristics of Qualitative Inheritance:

• A quantitative inheritance or monogenic inheritance deals with the inheritances of qualitative characters which have two contrasting expressions e.g. tall and dwarf pea plants.
• Each character is controlled by a single pair of contrasting alleles.
• There is no intermediate type.
• Each character has two distinct contrasting expressions i.e. they exhibit two distinct phenotypes.
• The degree of expression remains the same whether the character is controlled by one or both the dominant genes.
• Single effect genes are seen.
• It is not influenced by environmental factors.
• It shows a discontinuous pattern of inheritance.
• Individuals of F1 generation resembles the dominant parent.
• Individuals of the F2 generation are in the ratio 3:1. An intermediate expression is absent.
• It concerns with individual matings and their progeny.
• Analysis of this inheritance can be done by counting and finding ratios.
• Examples: Inheritances of qualitative characters like height, seed coat and seed colour of the pea plant.

Quantitative Inheritance:

A quantitative inheritance or polygenic inheritance deals with the inheritances of quantitative characters like height, weight, skin colour, intelligence, etc in the human population and exhibits continuous variation. Few characters in plants like height, the size, shape, number of seeds and fruits also exhibit quantitative inheritance.

In quantitative inheritance each gene has a certain amount of effect and the more number of dominant genes, the more is the degree of expression of the character. The gradation in the expression of the characters is determined by the number of gene pairs and all the gene pairs have an additive or cumulative effect.

Quantitative or polygenic inheritance was first studied by J. Kolreuter (1760) in case of height in tobacco and F. Galton (1883) in case of height and intelligence in human beings. Nilsson-Ehle (1908) obtained the first experimental proof of polygenic inheritance in case of kernel colour in wheat. The possible origin of polygenic inheritance is due to the duplication of a chromosome or its part, the increase in chromosomes number (Polyploidy) or the mutations producing genes having the similar effect.

Characteristics of Quantitative Inheritance:

• A quantitative inheritance or polygenic inheritance deals with the inheritances of quantitative characters.
• Each character is controlled by more than one pair of nonallelic genes (polygenes)
• In the case of one polygene pair, the number of phenotypes is 3 (1: 2: 1). In the case of two polygene pairs, the number of phenotypes is 5 (1: 4: 6: 4: 1). In the case of three polygene pairs,  the number of phenotypes is 7 (1 : 6: 15: 20: 15: 6: 1). Thus the number of intermediate types increases with the increase in the number of polygenes but the number of parental types remains the same
• Each character has an intergrading range of phenotypes.
• The degree of expression depends on the number of dominant genes.
• Single effect gene cannot be seen.
• It is influenced by environmental factors.
• It shows a continuous pattern of inheritance.
• F1 generation shows intermediate expression between the two parents.
• In F2 generation individuals with intermediate genotype and phenotype are maximum.
• It concerns with a population of organisms consisting of all possible kinds of matings.
• Analysis of this inheritance needs an appropriate statistical method and is complicated.
• Examples: Inheritances of quantitative characters like height, weight, skin colour, intelligence, etc in the human population. Few characters in plants like height, the size, shape, number of seeds and fruits also exhibit quantitative inheritance.

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Mendel performed his experiments with garden pea plant, which has traits or alleles having complete dominance and hence the laws of inheritance were proved. Other scientists performed their experiments on different plants and animals and found deviations to Mendelian ratios. Depending upon these experiments and observations, a different pattern of inheritance called gene interactions was discovered. This study is known as Post – Mendelian genetics or Neo-Mendelian genetics. In this article, we shall study the concept of pleiotropy and its effects.

The phenomenon of controlling more than one character at the same time is called pleiotropy or pleiotropism. Such genes are called pleiotropic genes. These genes produce more than one phenotypic effect which is totally unrelated. The pleiotropic effect is produced by a gene owing to a cascade (succession) of reactions during some metabolic pathway which is influenced by the original gene product and contributes to different phenotypic effects. The ratio is 2:1 instead of 3:1

Examples: In the pea plant, the same gene that affects the colour of the flower also influences the colour of the seed coat and the colour of the leaf axil. The gene that determines the size of the wings in Drosophila also affects its eye colour, the position of dorsal bristles, the shape of the spermatheca, fertility and length of life.

Effects of Pleiotropy in Human Beings:

Phenylketonuria (PKU):

Phenylketonuria also called PKU, is a rare inherited disorder that causes an amino acid called phenylalanine to build up in the body. PKU is caused by a defect in the gene that helps create the enzyme needed to break down phenylalanine.

Phenylketonuria is an autosomal recessive character controlled by a mutant gene present on the 12th chromosome. This mutant gene fails to code for the enzyme phenylalanine hydroxylase (PAH) required for normal metabolism of amino acid phenylalanine to tyrosine. Due to this, there is an accumulation of amino acid phenylalanine in the body fluids such as blood, sweat, and cerebrospinal fluid. An abnormal breakdown product phenyl ketone is found in urine. A higher level of phenylalanine and breakdown product phenyl ketone causes severe brain damage leading to mental retardation.

Symptoms: A musty odor in the breath, skin or urine, caused by too much phenylalanine in the body, neurological problems that may include seizures, skin rashes (eczema), fair skin and blue eyes, because phenylalanine can’t transform into melanin — the pigment responsible for hair and skin tone, abnormally small head (microcephaly), hyperactivity, intellectual disability, delayed development, behavioral, emotional and social problems, psychiatric disorders

Inheritance: For a child to inherit PKU, both the mother and father must have and pass on the defective gene. This pattern of inheritance is called autosomal recessive. If only one parent has the defective gene, there’s no risk of passing PKU to a child, but it’s possible for the child to be a carrier. Most often, PKU is passed to children by two parents who are carriers of the disorder but don’t know about it.

Marfan or Morphan’s Syndrome:

Marfan syndrome is a genetic disorder that affects the body’s connective tissue. Connective tissue holds all the body’s cells, organs and tissue together. It also plays an important role in helping the body grow and develop properly.

It is caused by a pleiotropic gene which is characterized by a slender body, limb elongation, hypermobility in joints, lens dislocation and a tendency to develop heart diseases. Marfan syndrome does not affect intelligence.

Marfan syndrome is caused by a defect in the gene that enables your body to produce a protein that helps give connective tissue its elasticity and strength. Most people with Marfan syndrome inherit the abnormal gene from a parent who has the disorder. In about 25 percent of the people who have the Marfan syndrome, the abnormal gene doesn’t come from either parent. In these cases, a new mutation develops spontaneously.

Sickle Cell Anaemia:

The gene Hb^s (recessive) is responsible for disease sickle cell anaemia. A normal or healthy gene is HbA .which is dominant. Thus disease carrier having heterozygotes HbA / Hbs show signs of mild anaemia as their R.B.C.s become sickle-shaped (half-moon) and their oxygen-carrying capacity decreases. But can live a normal life. But the homozygotes with recessive gene Hbs die of fatal anaemia. A gene which causes death of the bearer is called a lethal gene.  Two carrier parents will produce normal, carriers and sickle cell anemic children in 1:2:1 ratio.

Effect of the Pleiotropy in Mice:

Mice were first used for genetics research by the French biologist Lucien Cuénot in 1902. His breeding experiments showed that three mnemons (genes), allowed the production of one chromogen (pigment) and two distases (enzymes). The combination of the chromogen and one of the enzymes produced either a black or yellow colour in the mice. If there was no chromogen the mouse was albino. He showed mice inherited these coat colours in the ratio 3:1 as predicted by Mendel’s inheritance laws.

In 1905 Cuénot discovered the first lethal genetic mutation in the mouse. Lethal gene in mice causes death at an early stage of development, often before birth. The effect of the lethal gene is illustrated by the inheritance of fur (coat) color in mice,

In mice, yellow fur is dominant over non-yellow fur color. A cross was made between two heterozygous yellow fur mice (Yy and Yy) and F₁ generation was obtained. The cross is supposed to produce offsprings like 1 YY genotype, 2 Yy genotype, and 1 yy genotype.

The dominant homozygous organism with yellow fur color (YY) will never survive. The dominant homozygous organism dies in the embryonic stage because of a lethal combination. Hence the ratio 3:1 ratio changes to 2:1. It is a modified monohybrid ratio. Therefore, all living yellow fur mice are heterozygous(Yy). Here, gene ‘Y’ is recessive in relation to its effect on viability but dominant in relation to fur color.

Lethal Genes:

Sometimes, the pleiotropic gene effect may produce various abnormal phenotypic features which are collectively called syndromes.

If the effects of the pleiotropic gene become the cause of the death of an individual, then the pleiotropic gene is called the lethal gene. The lethal genes cause a great deviation from the normal development of an individual. Hence, that individual does not survive.  As a result of the lethal effect, Mendel’s monohybrid ratio of 3:1 gets modified and changed into 2:1. This lethal gene is seen either in the homozygous dominant condition or homozygous recessive condition.

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Mendel performed his experiments with garden pea plant, which has traits or alleles having complete dominance and hence the laws of inheritance were proved. Other scientists performed their experiments on different plants and animals and found deviations to Mendelian ratios. Depending upon these experiments and observations, a different pattern of inheritance called gene interactions was discovered. This study is known as Post – Mendelian genetics or Neo-Mendelian genetics. In this article we shall study the concept of complete dominance, partial dominance, and codominance.

Gene Interactions:

It was observed that the phenotypic expression of a gene can be modified or influenced by the other gene. This phenomenon is called gene interaction. There are two types of gene interactions

Intragenic (Interallelic) Interaction:

It occurs between alleles of the same gene e.g. incomplete dominance, co-dominance, and multiple alleles.

Intergenic (nonallelic) Interaction:

It occurs between the alleles of different genes on the same or different chromosomes. e.g. Pleiotropy, polygeny, epistasis, supplementary and complementary genes.

Neo-mendelian genetics includes interaction between alleles of a gene (interallelic interaction or intragenic interaction) and intergenic interaction or multiple genes inheritance.

Incomplete Dominance or Partial Dominance Or Blending Dominance:

In this case, both the genes of an allelomorphic pair express themselves partially. One gene cannot suppress the expression of other completely. e.g. four o’clock plant, snapdragon (Dog flower or Antirrhinum)

When a cross is made between true-breeding red-flowered plants (RR) and true breeding white-flowered plants (rr), the F1 generation is all pink-flowered (Rr) plants.

When pink-flowered plants of F1 generations are self-pollinated i.e. crossed among themselves, the F2 plants with red (RR), pink (Rr) and white (rr) flowers appear in the ratio 1:2:1. Here the phenotype ratio matches the genotype ratio of a monohybrid cross, but the phenotype ratio had changed from Mendelian ratio 3:1. No allele is dominant but the expression is intermediate between the two.

e.g. Andalusian Fowls (Chickens):

Andalusian fowls occur in three colours: black, white and blue. A cross between pure black (BB) and pure white (bb) produces F1 blue hybrid fowls. The genotype ratio of F1 generation is pure black (BB) : hybrid blue (bb) : Pure white (bb) is 1:2:1.

The phenomenon of incomplete dominance can be explained on the basis of Mendelian segregation. In complete dominance, the recessive factor cannot express, but in incomplete dominance both alleles have equal chance to express, hence we get hybrid intermediate in the F₁ generation.

Characteristics of Incomplete Dominance:

• In this case, both the genes of an allelomorphic pair express themselves partially.
• The expression of the hybrid genotype is intermediate to the phenotypes produced by each of the alleles separately.
• One gene can not suppress the expression of others completely.
• The expressed phenotype is new and no allele has its own effect.
• It is the result of the quantitative effect of alleles.
• The mixing of the phenotype effect of alleles is found.

Co-dominance:

In this case, both the genes of an allelomorphic pair express themselves equally in the F₁ generation. Such alleles express themselves independently even if present together in hybrids are called co-dominant alleles e.g. coat colour of cattle.

When red cattle are crossed with white cattle, the F1 generation has a roan coat colour where black and white patches appear separately. When F₁generation is self-crossed, F₂ Generation shows 4 phenotypes with ratio White: Both white and red: red = 1:2:1 and Genotypic ratio WW:RW: RR = 1:2:1. In F2 generation the phenotype ratio matches the genotype ratio.

Characteristics of Codominance:

• In this case, both the genes of an allelomorphic pair express themselves equally in the F₁ generation.
• Both the alleles express equally.
• The expressed phenotype is the combination of two phenotypes of the two alleles.
• No quantitative effect of alleles is found.
• No mixing of phenotype effect of alleles is found.

Complementary Genes:

In this type of interaction, two separate pairs of genes interact to produce the phenotype in such a way that neither dominant is expressive unless another one is present. Thus the effect of one dominant is expressed if and only if another dominant complement it. These types of genes are called complementary genes. This inheritance was discovered by W. Bateson and R. C. Punnett in sweet pea (Lathyrus odoratus).

When two certain white-flowered varieties of sweet pea are crossed with each other. They produced the F1 plant with red flowers. The F2 generation is obtained by self-pollination of the F1 generation, The ratio of red-flowered plants to white-flowered plants is found to be 9:7, which was different than the dihybrid ratio of 9:3:3:1.

The red colour in the flower of a sweet pea plant is produced by a pigment called anthocyanin. Its formation depends on two independent factors (C and P). Both these factors must be present to produce the pigment.

Multiple Alleles:

More than two alternative forms (alleles) of a gene in a population occupying the same locus on a chromosome or its homologue are known as multiple alleles. Thus the multiple alleles are multiple alternatives of the same gene which influence the same character and produce different expressions in different individuals of a species or population.

e.g. In Drosophila, a large number of multiple alleles are known. One of them is the series of wing abnormally ranging in size from normal wings to no wings.

Characteristics of Multiple Alleles:

• Multiple alleles arise by mutation of the wild type of gene.
• They occupy the same locus on homologous chromosomes.
• They regulate the same character but have a different degree of expression.
• They do not undergo crossing over.
• Only one member of the series of multiple alleles is present in a given chromosome and only two members in an individual.
• In multiple alleles, the wild type of expression is dominant while all other expressions are recessive to the wild type. But there may be complete dominance or codominance.

Blood Groups in Human Beings:

The gene I control ABO blood groups it has three alleles; IA, IB and i. The allele IA and IB produce slightly different types of sugar and allele i does not produce any sugar. The letter I is derived from the word Isoagglutinin (antigen)

As humans are diploid organisms, each person possesses any two of the three I genes. IA and IB are co-dominant and completely dominant on i.

Blood Group of Progeny (Children):

Science > Biology > Genetic Basis of Inheritance > Dominance and Codominance

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In this article, we shall study Mendel’s dihybrid cross experiment and its conclusions.

The first scientific explanation of inheritance was given by Mendel in 1866. He performed a series of experiments on garden pea in a scientific manner and proposed rules. which are called as Mendel’s Laws of Inheritance. His work is known as Mendelism. He laid down a foundation of Genetics hence he is called Father of genetics.

Dihybrid Cross (Two Factors Cross):

In an organism, there are many characters and each character is controlled by respective alleles. To study whether one pair of alleles affects or influences the inheritance pattern of a pair of other alleles, Mendel performed dihybrid cross experiments. In a dihybrid cross, he considered two traits simultaneously. Further Mendel performed trihybrid crosses and then he proposed the third law called the law of independent assortment.

A cross between two pure (homozygous) patterns in which the inheritance pattern of two contrasting characters is studied is called the dihybrid cross. It is a cross between two pure (obtained by true-breeding) parents differing in two pairs of contrasting characters.

He studied the inheritance of round and wrinkled characters of seed coat along with the yellow and green colours of seeds. He found that a cross between round yellow and wrinkled green seeds (P₁) produced only round yellow seeds in the F1 generation, but in F2 generation seeds of four phenotypes were observed. Two of these phenotypes were similar to the parental combinations (yellow round and green wrinkled), while the other two were new combinations (yellow wrinkled and green round).

The Procedure of Dihybrid Cross Experiment:

Step – 1: Selection of parents and obtaining Pure lines:

For dihybrid cross, Mendel selected pea plant having yellow and round seeds (YYRR) as the female parent and pea plant having green and wrinkled (yyrr) seeds as the male parent. He obtained pure line by selfing these plants for three generations. He confirmed that pea plant having yellow and round seeds are producing yellow and round seeds and pea plant having green and wrinkled seeds are producing green and wrinkled seeds.

Step – 2: Emasculation, Dusting and Raising F1 Generation:

Emasculation:

Emasculation is a process of removal of stamens before the formation of pollen grains (anthesis). This is done in the bud condition. The bud is carefully open and all stamens (9 + 1) are removed carefully.

Dusting and Raising F₁ Generation:

The pollens from the selected male flowers are dusted on the stigma of the emasculated female flower. This is an artificial cross. Mendel crossed many flowers, collected seeds and raised F1 generation. The female plant produces gametes with genes YR while male plants produced gametes with genes yr.

Yellow and round are dominant alleles, hence all F₁ Generation was with yellow and round seeds. All the plants produced in F₁ generation with yellow and round seeds (YyRr), which are heterozygous for both the alleles and are called dihybrid.

Punnett Square for F₁ Generation:

Step – 3: Selfing of F₁ hybrids to Produce F₂ Generation:

Mendel allowed natural pollination in each F₁ hybrid; collected seeds separately and F₂ generation is obtained.

Punnett Square for F₂ Generation:

Observations of the Dihybrid Cross Experiment:

Expectations:

Mendel expected the ratio of yellow and round seeds to green and wrinkled seeds to be 3:1.

Outcome:

• He found seeds of four types, yellow round, yellow wrinkled, green round and green wrinkled in the ratio 9:3:3:1.
• Out of these four types, two were parental combinations. Viz. yellow round and green wrinkled and two were new combinations like yellow wrinkled and green round.
• In all Mendelian dihybrid crosses the ratio in which four different phenotypes occurred was 9:3:3:1. This ratio is called the dihybrid ratio.
• Phenotypic ratio i.e. the ratio of the yellow round, yellow wrinkled, green round and green wrinkled in the ratio 9:3:3:1.

Mathematical Explanation of Mendel’s Law ofIndependent Assortment:

The meaning of the word assortment is ‘randomly and freely’. Thus probability theory is applicable to the dihybrid cross experiment. By the basic principle of probability, “Probability of two independent events occurring simultaneously is a product of their individual probabilities”

The probability of the first trait is 3:1 while that of the second trait is also 3:1. Thus the dihybrid ratio should be (3:1) x (3:1) = 3 x 3 : 3 x 1 : 1 x 3 : 1 x 1 i.e. 9:3:3:1 and Genotypic ratio YYRR: YYRr: YyRR: YyRr: Yyrr: Yyrr:yyRR:yyRr: yyrr is 1:2:2:4:1:2:1:2:1.

Mendel performed ample dihybrid crosses and reciprocal crosses with different combinations. Every time he got the same pattern of the result. The uniform expression was both dominant in F₁ generation. In F₂ generation always he got both dominant in large number.

Mendel’s Third Law of Inheritance (Law of Independent Assortment):

When two homozygous parents differing in two pairs of contrasting traits are crossed, the inheritance of one pair is independent of others. In other words, when a dihybrid (or polyhybrid) forms gametes, assortment (distribution) of alleles or different traits is independent of their original combinations in the parents. This law can be explained by help of dihybrid cross and dihybrid ratio.

It is immaterial whether both dominant characters enter the hybrid from the same or two different parents but the segregation and assortment remain the same. The appearances of new combinations prove the law. The law is universally applicable.

Importance of Mendel’s Laws:

• The concept of dominant and recessive factors is very important. This character is shown by many hereditary traits.
• It gives an idea of new combinations of traits which are very useful in developing a desirable trait in a progeny.
• This information is particularly used in the field of plant and animal breeding. Thus a new type of plants and animals can be produced by hybridization.

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In this article, we shall study Mendel’s monohybrid cross experiment and its conclusions.

The first scientific explanation of inheritance was given by Mendel in 1866. He performed a series of experiments on garden pea in a scientific manner and proposed rules. which are called as Mendel’s Laws of Inheritance. His work is known as Mendelism. He laid down a foundation of Genetics hence he is called Father of genetics.

Reasons for Selection of Garden Pea by Mendel:

• Garden pea is an annual plant and completes the life cycle within three or four months. Due to this short lifespan, he was able to take three generations in a year.
• It is a small herbaceous plant that produces many seeds and so he could grow thousands of pea plants in a small plot behind the church.
• It is naturally self-pollinating and was available in the form of many varieties with contrasting characters. There were no intermediate characters.
• Flowers are large enough for easy emasculation required for artificial cross and produce fertile offspring.

The Reason of Success of Mendel’s Experiment:

• Mendel studied the inheritance of one character at a time whereas earlier scientists had considered the organism as a whole. Initially, Mendel considered the inheritance of one trait only. (Monohybrid). Then he studied two traits together (dihybrid) and then three (Trihybrid).
• He started with pure line i.e. true breeding. He maintained a complete statistical record by counting an actual number of offspring.
• He carried out experiments up to the second and third generations.
• He conducted ample crosses and reciprocal crosses to eliminate chance.
• He dealt with a large sample size.

Monohybrid Cross:

A cross between two pure (homozygous) patterns in which the inheritance pattern of only one of contrasting characters is studied is called monohybrid cross. It is a cross between two pure (obtained by true breeding) parents differing in a single pair of contrasting characters. The procedure is as follows:

Step – 1: Selection of parents and obtaining Pure lines:

He selected pure line plants by ensuring that the selected male (pure dwarf) and female parent plants (pure tall) are breeding true for the selected trait or traits by selfing them for three generations. Thus pure line plants are homozygous for a given trait.

Step – 2: Emasculation, Dusting and Raising F₁ Generation (Hybridization):

Emasculation:

Emasculation is a process of removal of stamens before the formation of pollen grains (anthesis). This is done in the bud condition. The bud is carefully open and all stamens (9 + 1) are removed carefully. The stigma is protected against any foreign pollen with the help of a muslin bag.

Dusting and Raising F₁ Generation:

The pollens from the selected male flowers are dusted on the stigma of an emasculated female flower. The cross-pollinated flowers were enclosed in separate bags (bagging) to avoid further deposition of pollens from another source. During the pollination, it was assured that the pollen is mature and the stigma is receptive. This is an artificial cross. Mendel crossed many flowers, collected seeds and raised F1  generation. The plants used as parents are said to represent parental generation and are designated as P₁. The progeny obtained as a result of the crossing between parents is called the first filial (offspring) generation and is represented as F₁. All plants of F₁ generation were tall.

Punnett Square for F₁ Generation:

T (tall) is a dominant character t (dwarf) is a recessive character.

Collection and Separation of Seeds:

Seeds were separated and collected in marked bottles. To study characters of seeds they are studied immediately but for other characters, seeds were sown to raise next generation (F₂)of the plant.

Reciprocal Cross:

Mendel thought F₁ Generation is tall because tallness character is given by female parent and dwarfness character is given by a male parent.

To counter check it he performed reciprocal cross. Now, He selected pure line plants by ensuring that the selected male (pure tall) and female parent plants (pure dwarf) are breeding true for the selected trait or traits by selfing them for three generations.

He got the same result as in the first case. From this, he concluded that tallness is a dominant character, while the dwarfness is the recessive character.

The plants obtained from the crossing of two individuals differing at least one set of characters are known as hybrids and the process of obtaining them is called hybridization.

Punnett Square for (Reciprocal Cross) F₁ Generation:

T (tall) is a dominant character t (dwarf) is a recessive character.

Step – 3: Selfing of F1 hybrids to produce F2 Generation:

Mendel allowed natural pollination in each F₁ hybrid; collected seeds separately and F₂ generation (second filial) is obtained. The ratio of tall plants to dwarf plants in F₂ generation is found to be 3: 1

Punnett Square for F₂ Generation:

T (tall) is a dominant character t (dwarf) is a recessive character.

The ratio of tall plants to dwarf plants was around 3:1. Thus phenotype ratio (tall : dwarf) is 3: 1. The genotype ratio (pure tall : hybrid tall : pure dwarf) is 1:2:1.

Step – 4: Self Breeding:

Mendel carried self-breeding among F₂ generations and obtained F₃, then F₄ generations.

Checking With Other Traits:

Mendel performed monohybrid crosses and reciprocal crosses with all the seven pairs of contrasting characters separately and obtained similar results.

Only one of the two characters was expressed in F₁ generation. In F₂ generation the character which was shown in F₁ generation was in large number and the other in small number and the ratio was found to be 3:1. This ratio is called the monohybrid ratio.

Genotype Ratio for Monohybrid Cross:

The ratio of pure dominant character to hybrid character to pure contrasting recessive character is called the genotype ratio. In monohybrid cross experiment the genotype ratio for F₂ generation is 1:2:1.

Monohybrid Ratio for Monohybrid Cross:

Monohybrid ratio is defined as the phenotypic ratio of different types of offsprings (dominant and recessive) obtained in F₂ generation of a monohybrid cross. In monohybrid cross experiment the phenotype ratio for F₂ generation is 3:1.

Mendel’s Conclusions for Monohybrid Cross:

• Characters such as a height of a stem, a color of seed etc. are inherited separately as discrete particles or unit. He called them a factor or a determiner. Now it is called a gene.
• Each factor exists in contrasting or alternative forms. For e.g. for the height of a stem, there are two factors one for the tallness and other for the dwarfness. These two forms of genes are called alleles.
• One of the factors is dominant and another factor is recessive. The only dominant factor expresses in the F1 generation.
• In an organism, inheritance of each character is controlled by a pair of factors. One of the factors is contributed by the male parent and the other by the female parent. Thus higher organisms are diploid (2n)
• From F₂ generation Mendel concluded that in hybrid the two factors do not mix together but they just remain together.
• During gamete the formation, they separate or segregate and each gamete receives only one factor from each pair of factors. Thus gametes are haploid (n).

Diagrammatic Representation of Monohybrid Cross

Test Cross or Back Cross:

This is the method devised by Mendel to test the genotype of F₁ Hybrids. In F₁ generation 25% of plants are dwarf and we can definitely say that their genotype is ‘tt’ (Homozygous). But in the case of a tall plant, there are 25 % pure tall plants and 50% hybrid tall plants. Hence in the case of tall plants genotype can be ‘TT’ (Homozygous) or ‘Tt’ (Heterozygous). Thus we are not sure of the genotype of tall plants in the F₁ generation.

In a test cross, F1 hybrid is crossed with the homozygous recessive parent. Thus the offspring is crossed back with the parent, hence the test cross is also called a back cross.

If offspring has genotype (TT) then the F₂ generation obtained will be 100 % tall. It can be explained as follows. The recessive parent can produce only one type of gamete ‘t’, and the offspring of the first generation can produce only one type of gamete ‘T’. Thus the progeny (F₂ generation) will have genotype ‘Tt’ (tall).

If offspring has genotype (Tt) then the F₂ generation obtained will be 50 % tall and 50 % dwarfs. It can be explained as follows. The recessive parent can produce only one type of gamete ‘t’, while the hybrid of the first generation can produce two types of gametes ‘T’ and ‘t’. Thus half the progeny (F₂ generation) will have genotype ‘Tt’ (tall) and remaining half ‘tt’ (dwarf).

Diagrammatic Representation of Test Cross (With Flower Colour):

A test cross is a back cross but a back cross is not necessarily a test cross:

Case – 1: When the F₁ generation is crossed with Recessive Parent:

The recessive parent can produce only one type of gamete ‘t’, while the hybrid of the first generation can produce two types of gametes ‘T’ and ‘t’. Thus half the progeny (F₂ generation) will have genotype ‘Tt’ (tall) and remaining half ‘tt’ (dwarf).

Case – 2: When the F₁ generation is crossed with Dominant Parent:

The dominant parent can produce only one type of gamete ‘T’, while the hybrid of the first generation can produce two types of gametes ‘T’ and ‘t’. Thus 100 % progeny is tall. half the progeny will have genotype ‘TT’ (Pure tall) and remaining half ‘Tt’ (Hybrid tall).

A test cross is a cross used to find the genotype of F₁ generation. The test cross is a cross between an individual with the unknown genotype for a particular trait with a recessive plant for their trait, While back cross is a cross between an individual with the unknown genotype for a particular trait with a recessive or dominant plant for their trait.  Back cross can not indicate the genotype of F₁ generation. Hence a test cross is a back cross but a back cross is not a test cross.

The test cross method can be used to introduce useful recessive traits. Which is important in rapid crop improvement programmes.

Science > Biology > Genetic Basis of Inheritance > Mendel’s Monohybrid Cross Experiment

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