Terminology of Genetics

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

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