Question

In chickens, comb shape is determined by alleles at two loci. A walnut comb is produced when at least one dominant allele A is present at one locus and at least one dominant allele B is present at the second locus. A rose comb is produced when at least one dominant allele is present at the “A-locus” and two recessive alleles are present at the “B-locus”. A pea comb is produced when two recessive alleles are present at the “A-locus” and at least one dominant allele is present at the “B-locus”. When the genotype is a double homozygous recessive, a single comb is produced.   

Give the phenotypic ratios that will results from the following crosses: AaBb x aaBb

Answer 1

According to the problem,

Genotype for Walnut comb = AB

Genotype for Rose comb = Ab

Genotype for Pea comb = aB

Genotype for Single comb = ab

Gametes produced by AaBb are AB, Ab, aB, ab.

Gametes produced by aaBb are aB and ab.

Punnett square

	AB	aB	Ab	ab
aB	AaBB	aaBB	AaBb	aaBb
ab	AaBb	aaBb	Aabb	aabb

According to the table;

Walnut comb : AaBB, AaBb = 3

Rose comb : Aabb = 1

Pea comb : aaBB, aaBb = 3

Single comb : aabb = 1

Therefore, the phenotypic ratio is Walnut comb : Pea comb : Rose comb : Single comb = 3 : 3 : 1 : 1