Genetics

The 9:3:3:1 ratio is the phenotypic ratio in F2 from a dihybrid cross (AaBb × AaBb) with two independently assorting gene pairs. 9/16 show both dominant traits, 3/16 show dominant A only, 3/16 show dominant B only, 1/16 show both recessive traits. This ratio is modified by epistasis — e.g., duplicate dominant epistasis gives 15:1, recessive epistasis gives 9:3:4, dominant epistasis gives 12:3:1.

The Hardy-Weinberg law states that allele and genotype frequencies in a large, randomly mating population remain constant across generations in the absence of mutation, selection, gene flow, and genetic drift. If allele frequencies are p (dominant) and q (recessive), genotype frequencies are p² (AA) + 2pq (Aa) + q² (aa) = 1. It is used to calculate carrier frequencies in population genetics — tested in ICAR JRF.

Linkage occurs when two genes are located on the same chromosome and tend to be inherited together rather than independently (violating Mendel's Law of Independent Assortment). Complete linkage produces parental combinations only; incomplete linkage allows some recombinants via crossing over. Recombination frequency (cM) = (recombinant offspring / total offspring) × 100. Genes >50 cM apart behave as if unlinked.

Epistasis is the masking of one gene's effect by another gene (not alleles). Types and F2 ratios: Dominant epistasis (12:3:1), Recessive epistasis (9:3:4), Duplicate dominant epistasis (15:1), Duplicate recessive epistasis (9:7), Dominant and recessive epistasis (13:3). Recessive epistasis (9:3:4) occurs in flower colour in sweet pea — classic exam example.

Inbreeding depression is the reduction in fitness and vigour that results from repeated self-fertilisation or mating between related individuals. It occurs because inbreeding increases homozygosity, exposing harmful recessive alleles in the homozygous state. Degree of inbreeding is measured by the Inbreeding Coefficient (F). Heterosis (hybrid vigour) is the opposite — F1 hybrids exceed both parents in performance.

Quantitative inheritance involves traits controlled by multiple genes (polygenes) each with small, additive effects — also called polygenic or continuous variation. Examples: plant height, yield, grain weight, oil content. Such traits show a normal distribution in populations. Heritability (h²) measures how much of the phenotypic variation is due to additive genetic effects — key for predicting selection response in plant breeding.