Genetic Terminology

Genetic Terminology

Character: It is a specific feature of an organism e.g. height of stem.

Trait: An inherited character and its detectable variant e.g. Tall or dwarf.

Factor: It is a unit of heredity, a particle present in the organism which is responsible for the inheritance and expression of a character. Factor determines a genetical (biological) character of an organism.

Gene : It is a particular segment of DNA which is responsible for the inheritance and expression of that character.

Alleles or Allelomorphs : The two or more alternative forms of a given gene (factor) are called alleles of each other. They occupy identical loci (positions) on homologous chromosomes. Allele is a short form of Allelomorph.

Dominant :

allele that expresses its trait even in the presence of an alternative allele i.e. in heterozygous condition only. Alternatively, the allele that expresses in F1 is called dominant. (It is an allele of a pair that masks the expression of other allele in F1 generation.)

Recessive

Allele is not expressed in the presence of an alternative allele (in heterozygous condition). It expresses only in the presence of another identical allele. It is an allele that does not express in F1 hybrid

Phenotype

Is external apperance of an individual for any trait is called phenotype for that trait.

It is observable and is determined by different combinations of alleles.

e.g.  In pea, for the height of stem (plant) tall and dwarf are the two phenotypes (Tall is determined by TT or Tt and dwarf by tt).

Genotype :

Genetic constitution or genetic makeup of an organism with respect to a particular trait.

It is representation of the genetic constitution of an individual with respect to a single character or a set of characters. e.g. pea tall plants can have genotype TT or Tt and dwarf has tt

 Homozygous (pure)

individual possessing identical allels for a particular trait, is called homozygous or pure for that trait.

Breeds are true and produces only one type of gametes e.g., tall with TT and dwarf with tt.

 Heterozygous

Individual  possessing different  alleles for a particular trait, is called heterozygous.

Does not true breed and produces two types of gametes e.g. F1 generation hybrids (Tt).

Also  called hybrid.

 Pure line

An individual or a group of individuals (population) which is homozygous or true breeding for one or more traits, constitutes pure line i.e. plant which breeds true for a particular character.

It is a descendent of a single homozygous parent produced after self fertilization.

Monohybrid :

It is heterozygous for one trait

produced from a cross between two pure parents differing in single pair of contrasting characters

e.g. Hybrid tall produced in a cross between pure tall and pure dwarf parents.

It is a heterozygote for a single pair of alleles.

F1 generation

First filial generation.

Consists of all off-springs produced from a parental cross.

F2 generation

Second filial generation.

Progeny produced from a cross between two F1 individuals (e.g. Tt × Tt).

The second generation (progeny) produced by selfing (inbreeding) of F1 generation offsprings.

Punnett square/checker board:

Probability table representing different permutations and combination of fertilization between gametes of the opposite mating types.

It  is a diagrammatic representation of a particular cross to predict the progeny of a cross.

Homologous Chromosomes:

Morphologically, genetically and structurally essentially identical chromosomes present in a diploid cell, are called homologous chromosomes

Such chromosomes synapse during meiosis.

Back cross :

It is a cross of F1 progeny with any of the parents

(e.g. F1 tall × pure tall; F1 tall × pure dwarf i.e. Tt × TT/tt).

Test cross:

A cross of F1 progeny with homozygous recessive parent (e.g. F1 tall × pure dwarf i.e. Tt × tt).

Used  to test the homozygous/ heterozygous nature of hybrid. It is a kind of back cross.

Phenotypic ratio :

Ratio of offsprings produced in F2 generation with respect to their physical appearance

e.g. 3 Tall : 1 dwarf, is F2 ‘Phenotypic ratio’ in monohybrid cross.

Genotypic ratio

It is the ratio of the offsprings produced in the F2 generation with respect to their genetic makeup

e.g. 1 TT: 2Tt: 1 tt,  is F2 genotypic ratio in monohybrid cross.

Monohybrid cross:

A cross between parents differeing in only one heritable trait is called monohybrid cross.

e.g. cross of pure tall and pure dwarf plants.

Mendel performed the monohybrid cross between two pea plants with only one pair of contransting character.

Dihybrid cross :

A cross between parents differing in two heritable traits, is called dihybrid cross

e.g. cross of pure tall, round seeded plant with dwarf, wrinkled seeded plant.

Mendel also performed the dihybrid cross between pea plants that differed in two pairs of contrasting characters.

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