Question

Most black bears (Ursus americanus) are black or brown in color. However, occasional white bears of this species appear in some populations along the coast of British Columbia. Kermit Ritland and his colleagues determined that white coat color in these bears results from a recessive mutation (g) caused by a single nucleotide replacement in which guanine substitutes for adenine at the melanocortin-1 receptor locus (mcr1), the same locus responsible or red hair in humans. (K. Ritland, C. Newton, and H.D. Marshall. 2001. Current Biology 11: 1468-1472). The wild type allele at this locus (G) encodes the black or brown color. Ritland and his colleagues collected samples from bears on three islands and determined their genotypes at the mcr1 locus.

Genotype	number
GG	42
Gg	24
gg	21

a. What are the frequencies of the G and g alleles in these bears?

b. Give the genotypic frequencies expected if the population is in Hardy-Weinberg equilibrium.

Answer 1

The total number of the population = 42+24+21 = 87

a. To find the frequency of G and g alleles the following formula is used:

frequency of G allele = {(number of GG population) + [(1/2)*(number of Gg population)]}/ total number of population

frequency of G allele = 42+[(1/2)*(24)]/87 = 42+12/87 = 54/87 = 0.62

frequency of g allele = {(number of gg population) + [(1/2)*(number of Gg population)]}/ total number of population

frequency of g allele = 21+[(1/2)*(24)]/87 = 21+12/87 = 33/87 = 0.38

Hence the frequency of G = 0.62 and that of g allele = 0.38

b. To find the genotypic frequencies expected in the population is:

Genotypic frequency of GG = (G allele frequency)² = (0.62)² = 0.38

Genotypic frequency of Gg = 2* G allele frequency * g allele frequency = 2*0.62*0.38 = 0.47

Genotypic frequency of gg = (g allele frequency)² = (0.38)² = 0.15

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