Question

The HAIR gene (abbreviated as H) controls hair color with the dominant phenotype being dark brown hair and the recessive phenotype being blonde hair. The WIDOW gene (abbreviated as W) controls the presence of a ‘widows peak’ in the hairline (having a point at the center of the hairline). Having a widows peak is dominant and not having a widows peak is recessive. Gene H and Gene W are linked genes. Two individuals with the genotypes indicated below mate (note: upper-case letters indicate dominant alleles and lower-case letters indicate recessive alleles).

Hw / hW   x   hw / HW  

A.  From this mating, what proportion of the offspring will be heterozygous for both genes?

1. 1/4

2. 3/4

3. 9/16

4. 3/16

5. 0

B.  From this mating, what proportion of the offspring will be heterozygous for just the WIDOW gene (and homozygous for the HAIR gene)?

1. 1/4

2. 0

3. 9/16

4. 3/4

5. 1/2

Answer 1

given that these two genes are linked but the distance between these genes are not given, assuming both genes are completely dominant.

Hw / hW   x   hw / HW  

Hw / hW produce two types of gametes, Hw and hW ( H and w alleles are on one chromosome and h and W alleles are on another chromosome) and hw / HW  produce hw gamete and HW gamete, ( h and w alleles are on one chromosome and H and W alleles are on another chromosome).

Hw / hW * hw/HW

	Hw	hW
hw	Hw/hw	hW/hw
HW	Hw/HW	hW/HW

a) none of the progenies are heterozygous for both traits, so the proportion of progenies who are heterozygous for both traits=0

5) 0.

b) proportion of the offspring will be heterozygous for just the WIDOW gene = number of progeny heterozygous for only W gene/total number of progenies=2/4=1/2

5) 1/2