3.2 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 is passed from one generation to the next through gametes). Factor determines a genetic (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 :

It is an 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 :

This 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 :

The external appearance 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 make up 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) :

An individual possessing identical alleles for a particular trait, is called homozygous or pure for that trait. Homozygous breeds true to the trait and produces only one type of gametes e.g., tall with TT and dwarf with tt.

Heterozygous :

An individual possessing contrasting alleles for a particular trait, is called heterozygous. Heterozygous does not breed true for that trait and produces two types of gametes e.g. F1 generation hybrids (Tt). Heterozygous individual is 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 and is 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 :

It refers to the first filial generation. It consists of all off-springs produced from a parental cross. Alternatively, it is first generation from a given mating between pure parents having contrasting characters.

F2 generation :

The second generation (progeny) produced by selfing (inbreeding) of F1 generation offsprings is called second filial generation. e.g. Progeny produced from a cross between two F1 individuals (e.g. Tt × Tt).

Punnett square/checker board :

It is a probability table representing different permutations and combination of fertilization between gametes of the opposite mating types. In short, it is a diagrammatic representation of a particular cross to predict the progeny of a cross.

Homologous Chromosomes :

The 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 :

It is a cross of F1 progeny with homozygous recessive parent (e.g. F1 tall × pure dwarf i.e. Tt × tt). It is used to test the homozygous/ heterozygous nature of hybrid. It is a kind of back cross.

Phenotypic ratio :

It is the ratio of the offsprings produced in F2 and subsequent 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 and subsequent generation with respect to their genetic make up e.g. 1 TT : 2Tt : 1 tt, is F2 genotypic ratio in monohybrid cross.