Principles of Inheritance and Variation

Characters are controlled by discrete units called factors, Factors occur in pairs, In a dissimilar pair of factors, one dominates the other

Mendel's Law of Dominance states that: (1) Characters are controlled by discrete units called factors (genes), (2) Factors occur in pairs (alleles), and (3) In a dissimilar pair, one factor dominates the other. Factors do not blend and individuals have two factors for each trait.

F1 shows a phenotype intermediate between both parents, Neither allele is completely dominant over the other, Example: Red flowers × White flowers = Pink flowers in F1

In incomplete dominance, neither allele is completely dominant, resulting in F1 offspring with an intermediate phenotype. The classic example is red × white flowers producing pink flowers in snapdragons, unlike complete dominance where F1 resembles one parent.

Type AB

When both IA and IB alleles are present (genotype IAIB), both alleles are co-dominant and express their respective antigens. This results in AB blood type, where both A and B antigens are present on red blood cells.

9:3:3:1

In a dihybrid cross F2 generation, the phenotypic ratio is 9:3:3:1. This represents 9 individuals with both dominant traits, 3 with first dominant and second recessive, 3 with first recessive and second dominant, and 1 with both recessive traits.