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.

Science > Biology > Genetic Basis of Inheritance > Polygenic 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.

Science > Biology > Genetic Basis of Inheritance > Pleiotropy

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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.

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

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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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In this article, we shall study the terminology of Genetics and understand the concept of alleles.

Heredity:

The transmission of characters from one generation to the next, that is from parents to offsprings (progeny) is known as heredity.

These hereditary characters are present on the chromosomes in the form of genes. These gene combinations express characters which may be more similar to one of its two parents. The differences in characters of offspring mainly depend upon a unique process of crossing over that occurs during meiosis. This is one of the main reasons for producing recombination.

Inheritance:

Inheritance is the process by which characters are passed on from parent to progeny. it is the basis of heredity. Inheritance studies both the similarities and variations.

Variations:

Variation means differences between parents and their offsprings or between offsprings of same parents or between members of the same population (same species).

The above picture shows two variations in species elephant (Asian elephant and African elephant). They are the same species but there is variation in their size, structure etc. due to their adaptations to their habitats.

Causes of Variations:

• It arises due to mutation or sudden change in the genes.
• At fertilization, there is a random mixing of paternal and maternal chromosomes with different gene combinations. Such a source of variation which is most common is called genetic recombination. The variation arises because genes get shifted and exchanged during meiosis at the time of formation of gametes, giving rise to new gene combinations.
• Heritable Variations generally arise because of mutation and recombination.

Importance of Variation:

It has survival value for the population because if the environment changes, some individuals (variants) may be able to adapt to new situations and save the population from dying out (extinction).

Genetics:

Genetics is a branch of biology that deals with heredity and variations. This term was coined by William Bateson in 1906. Word Genetics is derived from the Greek word “Genesis”, which means ‘to grow into’.

Clones:

Organisms produced by asexual reproduction are exact copies of their parents. They are carbon copies of each other and of parents. They are called ramets. The group of identical individuals produced from the single parent is known as a clone. Animals produced by asexual reproduction or plants (budding, fission, spore formation, grafting, and layering) produced by vegetative propagation are identical to their parents and hence are clones. They can be made in the lab.

Clones are organisms that are exact genetic copies of each other because every single bit of their DNA is identical.

Characteristics of Clone:

• It is formed by asexual reproduction
• It is produced from a single parent. Hence it is mono-parental.
• It is formed by mitotic cell division.
• As meiosis is absent, recombination of genes does not occur.
• They are carbon copies of each other and of parent both in genotype and phenotype.
• Members of a clone are genetically identical.

Offspring:

Organisms produced by sexual reproduction are called offspring and they are not identical to either of their parents but inherits some of the characteristics of both the parents.

Characteristics of Offspring:

• They are produced by sexual reproduction
• It is produced by two parents (a male and a female). Hence it is biparental.
• It is formed by the fusion of the male and the female gametes produced by meiosis.
• Due to meiosis, recombination of genes takes place.
• Offspring differ from each other and also from parents.
• Offspring have differences in their genotype.

Factor:

It is a unit of heredity. This concept was given by Mendel. The unit of inheritance and expression of a particular character is controlled by inheritable units called factor (gene) which are present in pairs in parental cells and singly in the gametes. It is responsible for the inheritance and expression of character.

Gene:

It is a particular segment of DNA which is responsible for the inheritance and expression of that character. Johannsen used the term gene for the first time. Each gene has information for the synthesis of a particular polypeptide.

Character:

It is a feature of the organism. e.g. Height of stem or height of a person, flower colour, seed shape, eye colour, skin colour, etc. A character is a feature of the organism or external appearance of the organism,

Trait:

The trait is the morphologically or physiologically visible character, e.g. colour of a flower, and shape of the seed. It is an inherited character and its detectable variant. e.g. Tall or dwarf.

The trait is inherited character and it’s a detectable variant. For e.g consider the height of a plant, it is external appearance hence height his character or feature. Now it has two variants, tall or dwarf, then tall and dwarf are their traits. Traits are in multiple forms and environmentally determined.

Alleles or Allelomorphs:

The two (or more) alternative forms of a gene (factor) are called alleles of each other. They occupy identical positions on homologous chromosomes.

For example in the pea plant, the gene for producing seed shape may occur in two alternative forms: smooth (S) and wrinkled (s). Genes for smooth wrinkled seeds are alleles of each other and occupy the same locus on homologous chromosomes.

Homologous and Heterologous Chromosomes:

The term homozygous and heterozygous were coined by Bateson and Saunders (1902) for the types of symbolised gene combinations.

The morphologically and structurally similar chromosomes present in a diploid cell are called homologous chromosomes or homologues. The chromosomes of true breeding tall (TT) and true breeding dwarf (tt) pea plants are homozygous.

If the two members of an allelic pair are not the same, then the individual chromosome is called heterozygous or heterologous. The chromosomes of the next generation of crossing between the true breeding tall (TT) and true breeding swarf (tt) pea plants can be tall (Tt) which is heterozygous.

Dominant of Allele or Dominant Trait:

It is an allele that expresses its trait even in the presence of an alternative allele. Out of the two alleles or allelomorphs of a trait, the one which expresses itself in a heterozygous organism in the F1 hybrid is called the dominant trait (dominant allele).

If the allelic combination in an organism is Tt, and T (tallness) expresses itself but t (dwarfness) cannot, so T is the dominant allele, and tallness is dominant on dwarfness.

Recessive of Allele Recessive Trait:

It is an allele which is not expressed in the presence of an alternative allele. i.e. in the heterozygous condition. Out of the two alleles or allelomorphs of a trait, the one which remains masked in the F1 individual but gets expressed in the next generation (F2), is called recessive.

If the allelic combination in an organism is Tt (heterozygous), and T (tallness) expresses itself but t (dwarfness) being recessive cannot express itself, so T is the dominant allele, and tallness is dominant on dwarfness. Recessive allele does express itself only in the homozygous state (e.g. tt).

Genotype:

 It is a representation of the genetic constitution of an individual with respect to a single character or a set of characters. Genotype represents the genetic makeup or gene complement of an organism with respect to one or more characters. It represents the genes which it receives from the two parents.

In garden pea plant, the genotype for a tall plant is TT or Tt. For a dwarf plant, the genotype is tt. If we consider seed of garden pea plant, then the smoothness of coat of seed can be considered as a character. The genotype of a pure smooth seeded parent pea plant is SS and it will always breed true for the smooth-seeded character, but plants having Ss on selfing would give rise to a population represented by 3: 1 ratio for smooth seeded plants and wrinkled seeded plants.

Characteristics of Genotype

• It is a gene complement of an organism with respect to one or more characters.
• Individuals with different genotype can have the same or different phenotype.
• It is not influenced by the phenotype.
• It is not affected by the environment or age.
• The genotype of an individual can be obtained by performing specific experiments.

Phenotype:

The external appearance (morphological or physiological characters) of an individual for any trait is called phenotype for that trait. e.g. Smooth-seeded shape or wrinkled shape of seeds represent two different phenotypes. The phenotype is the external manifestation of the genotype of an organism. It is an interaction between the genotype and the environment. Phenotype changes to some extent with environment and age.

In garden pea plant Height of a stem is a character. Its two variants are tall (T) and dwarf (t). Thus tall and dwarf are phenotypes.

Characteristics of Phenotype

• It is the external appearance (morphological or physiological characters) of an individual for any trait
• Individuals with different phenotype generally have different genotype.
• It is the expression of the genotype.
• It is affected by the environment or age.
• The phenotype of an individual can be obtained from direct observations.

Homozygous:

An individual possessing similar alleles for a particular trait is called homozygous or pure for that trait. They produce only one type of gamete. e.g. Parental tall with TT and dwarf plant with tt are homozygous.

Characteristics of Homozygous:

• The alleles of a pair are identical in a homozygous individual.
• They possess either both dominant (TT) or both recessive alleles (tt).
• Homozygous individuals may be homozygous dominant or homozygous recessive.
• They are pure for the trait.
• Gametes produced by the homozygous individuals are identical. Thus only one type of gametes is produced.
• On self-breeding, they produce only one type of offspring.

Heterozygous:

An individual possessing dissimilar alleles for a particular trait is called heterozygous or hybrid for that trait. They produce two types of gamete. e.g. F1 Generation of hybrid Tt.

Characteristics of Heterozygous:

• The alleles of a pair are different in a heterozygous individual.
• They possess one dominant allele and one recessive allele.
• Heterozygous individuals show only of one type.
• They are not pure for the trait.
• Two types of gametes are produced.
• On self-breeding, they produce three types of genotypes: homozygous dominant, homozygous recessive and heterozygous.

Pure Line:

An individual or a group of individuals (population) that is homozygous or true breeding for one or more trait. The pure line is obtained by repetitive self-fertilization or breeding between homozygous identical ancestors. Offspring of the pure line are exactly identical. The term pure line was coined by Johannsen.

Hybridisation:

Crossing organisms belonging to different species for getting desirable qualities in the offspring is called hybridisation.

Hybrid:

It is a heterozygous individual produced from any cross involving pure parents having one or more contrasting traits. If the pure tall plant (TT) is crossed with a pure dwarf plant (tt), the progeny is hybrid tall (Tt).

Monohybrid:

It is heterozygous for one trait and produced in a cross between two pure parents differing in a single pair of contrasting character. e.g. Hybrid tall produced in a cross between pure tall and dwarf parent.

Monohybrid Cross:

It is a cross that involves the study of the inheritance of only one pair of contrasting character at a time. The inheritance of tall and dwarf characters is an example of a monohybrid cross.

Dihybrid:

It is heterozygous for two traits and produced in a cross between two pure parents differing in two pairs of contrasting character.

e.g. Hybrid tall produced in a cross between pure tall bearing white flowers and dwarf parent bearing purple flowers.

Dihybrid Cross:

It is a cross that involves the study of the inheritance of two pairs of contrasting characters at a time. The inheritance of yellow round seed character and green wrinkled character is an example of a dihybrid cross.

Poly-hybrid Cross:

It is a cross that involves the study of the inheritance of more than two pairs of contrasting characters at a time.

F₁ Generation:

The progeny (offspring) produced from a cross is called the first filial or F₁ generation. It shows uniform expression.

F₂ Generation:

The second-generation (progeny or offspring) produced from selfing (interbreeding) of F₁ generation offspring is called second filial or F₂ Generation. It shows two or more types of individual in particular.

Back Cross:

When F₁ hybrids are crossed with either of the parents or parental type, then such a cross between the offspring and the parents is known as the back cross.

Test Cross:

The test cross is a cross between a heterozygous F₁ hybrid and double recessive homozygous. The test cross is used to determine whether the individuals exhibiting dominant character are homozygous or heterozygous.

Genome:

Genome is the total number of genes present in the haploid set of chromosomes. A gamete is haploid and contains only one set of the genome.

Punnett Square (Checker Board):

It is a diagram that is used to show possibilities of combinations in particular cross or breeding experiment. It helps us to know possible genotypes and phenotypes of offspring produced in the cross.

Science > Biology > Genetic Basis of Inheritance > Terminology of Genetics

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