In humans, the presence of chin and cheek dimples is dominant to the absence of
ID: 49496 • Letter: I
Question
In humans, the presence of chin and cheek dimples is dominant to the absence of dimples, and the ability to taste the bitter compound PTC (phenylthiocarbamide) is dominant to the inability to taste this compound. Both traits are under the control of autosomal genes that are unlinked.
A population from Ghana is examined, and the following allele frequencies are calculated.
Dimples
Frequency
Tasting
Frequency
D
0.62
T
0.76
d
0.38
t
0.24
a. Determine the frequency of genotypes for each gene.
b. What are the expected frequencies of the four possible phenotype combinations:
dimpled tasters, undimpled tasters, dimpled nontasters, and undimpled nontasters
Dimples
Frequency
Tasting
Frequency
D
0.62
T
0.76
d
0.38
t
0.24
Explanation / Answer
a). For a gene that exists in two alleles, the Hardy–Weinberg equation states that (p2) + (2pq) + (q2) = 1
F(DD) = p2 (genotype frequency of homozygotes)
F(Dd)=2pq (genotype frequency of heterozygotes)
F(dd)=q2 (genotype frequency of homozygotes)
If (D)p=0.62 and (d)q=0.38, then
p2 = (0.62)2 = 0.3844
2pq = 2×(0.62)×(0.38) = 0.4712
q2 = (0.38)2 = 0.1444
If the allele frequency of D is 62% and allele frequency d is 38% the the expected genotype frequency of DD is 38.44%, Dd is 47.12% and dd is 14.44%
If (T) p=0.62 and (t)q=0.38, then
p2 = (0.76)2 = 0.5776
2pq = 2×(0.76)×(0.24) = 0.3648
q2 = (0.38)2 = 0.0576
If the allele frequency of T is 76% and allele frequency t is 24% the the expected genotype frequency of DD is 57.76%, Dd is 36.48 and dd is 0.057%
b). Possible phenotype combinations
Dimple tasters Undimple nontasters
DDTT X ddtt
F1 generation DdTt
F1 selfed DdTt x DdTt
F2 generation Phenotypic combinations
Dimple tasters =9
undimple tasters =3
Dimple non tasters =3
Undimple nontasters =1