In humans, the presence of chin and cheek dimples is dominant to the absence of
ID: 48876 • 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
Hardy and Weinberg also described all the possible genotypes for a gene with two alleles. The binomial expansion representing this is, p2 + 2pq + q2 = 1.0
Where,
p2 = proportion of homozygous dominant individuals
q2 = proportion of homozygous recessive individuals
2pq = proportion of heterozygotes.
a.) The frequency of homozygous dominant allele carrying individuals (DD) = 0.62* 0.62 = 0.3844
The frequency of heterozygous allele carrying individuals (Dd) = 2*0.62* 0.38 = 0.4712
The frequency of homozygous recessive allele carrying individuals (dd) = 0.38* 0.38 = 0.1444.
In case of taste genes,
The frequency of homozygous dominant allele carrying individuals (TT) = 0.76* 0.76 = 0.5776
The frequency of heterozygous allele carrying individuals (Tt) =2* 0.76* 0.24 = 0.3648
The frequency of homozygous recessive allele carrying individuals (tt) = 0.24* 0.24 = 0.0576.
b.) The frequency of individual with dimples = 0.3844+0.4712 = 0.8556
The frequency of individual without dimples = 0.1444
The frequency of individual with taste for PTC = 0.5776+ 0.1824 = 0.944
The frequency of individual without taste for PTC = 0.0576