Inheritance & Variation

Terminology

i. Character: A visible feature of an organism is a character. e.g. Height, seed colour.

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

iii. 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).

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

v. Alleles or Allelomorphs: Two or more alternative forms of a gene present at the same loci (positions) of homologous chromosomes and controlling the same character are called as alleles or allelomorphs.

vi. Dominant: It is an allele that expresses its trait even in the presence of an alternative allele i.e. in heterozygous condition only.

OR

The allele that expresses in F, is called dominant.

vii. Recessive: The character which is not expressed in F₁ generation is called recessive character.

OR

It is an allele which is not expressed in presence of an alternative allele. Recessive allele expresses in homozygous condition, i.e. in the presence of another identical allele.

viii. Phenotype: The external appearance of an individual for a given trait. e.g. Tallness, dwarfness.

ix. Genotype: It is the genetic constitution of an individual with respect to a single character or a set of characters. e.g. Tall (TT or Tt), Dwarf (tt).

X. Homozygous (pure): An individual having identical alleles for a particular character is homozygous for that character. It is pure or true breeding for that trait. e.g. TT, tt.

xi. Heterozygous: An individual possessing contrasting (dissimilar) alleles for a particular trait is called heterozygous. It is a hybrid and does not breed true for that trait. e.g. Tt

xii. 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.

xiii. Monohybrid: It is heterozygous for one trait and is produced from a cross between two pure parents differing in a single pair of contrasting characters e.g. Hybrid tall (Tt) produced in a cross between pure tall (TT) and pure dwarf (tt) parents.

xiv. F₁ generation: It refers to the first filial generation. It consists of all offsprings produced from a parental cross.

OR

It is first generation from a mating between pure parents having contrasting characters.

XV. F2 generation: The second generation (progeny) produced by selfing (inbreeding) of F₁ generation offsprings is called second filial generation.

xvi. Punnett square/checker board: It is a probability table representing different permutations and combination of fertilization between gametes of the opposite mating types.

xvii. Phenotypic ratio: It is the ratio of the offsprings produced in F2 and subsequent generation with respect to their physical appearance.

xviii. Genotypic ratio: It is the ratio of the offsprings produced in the F2 and subsequent generation with respect to their genetic make-up.

xix. Homologous chromosomes: The morphologically, genetically and structurally identical chromosomes present in a diploid cell, are called homologous chromosomes.