Question

Explain the law of dominance and compare how it differs from incomplete dominance and co-dominance.

Answer 1

Law of Dominance: “When two homozygous individuals with one or more sets of contrasting characters are crossed, the alleles (characters) that appear in F₁ are dominant and those which do not appear in F₁ are recessive”.

1. Incomplete Dominance: In the incomplete dominance, both the alleles (genes) of an allelomorphic pair express themselves partially. e.g.: The flower colour of Mirabilis jalapa.

2. Co-dominance: In co-dominance, both the alleles (genes) of an allelomorphic pair express themselves equally in F₁ hybrids. Such alleles which are able to express themselves equally independently in hybrids are called co-dominant alleles. Thus in co-dominance, both alleles are expressed. e.g.: Coat colour in cattle.

Chart of Dominance:

Second Generation  \[\ce{\underset{Tall}{Tt} × \underset{Tall}{Tt}}\]

Male gametes →	T	t
Female gametes ↓
T	TT	Tt
t	TT	tt

Phenotypic Ratio - \[\ce{\underset{Tall}{3}: \underset{Dwarf}{1}}\]

Genotypic Ratio

TT -	1
Tt -	2	1 : 2 : 1
tt -	1