At the C locus in horses, chestnuts (sorrels) are CC, palominos are Cc cr , and

Question

At the C locus in horses, chestnuts (sorrels) are CC, palominos are Cccr, and cremellos are ccr ccr. Assume we have two large populations of horses in which stallions of one herd are bred to mares of the other herd, and vice versa. Coat color is not a factor in selection and/or mating decisions. The frequency of the C allele in population 1 is 0.70, while the frequency of the C allele in population 2 is 0.25.

a. What will be the gene and genotypic frequencies at the C locus in the offspring (F1) population?

b. If the F1 offspring are mated among themselves, what will be the gene and genotypic frequencies in the F2 generation (assuming HW equilibrium)?

Explanation / Answer

Answer a) Let us put the phenotype and genotype in a tabular form

Let us now draw the cross in the table below where the Stallion of Population 1 were bred with mares of Population 2 and vice versa.

SPERM

                                                           EGGS

C

C

ccr

CC

Cccr

ccr

Cccr

ccrccr

Genotypes         

CC

Cccr

ccrccr

So the Genes are CC Cccr and ccrccr and the genotypic ratio is of 1:2:1 in the F1 generation.

The frequency of C allele in population 1 is 0.7. This means that 70% of the population has the C allele.

Now according to Hardy -Weinberg Equilibrium,

(p + q)2  = 1

or, p + q = 1

or, p2 + 2pq + q2 = 1

Now, if we assume that C allele represents p, then q represents ccr whose frequency is 1-0.7 = 0.3

So, the genotypic frequencies in the F1 populations would be CC (p2) = (0.7)2 = 0.49,

Cccr (2pq) = 2*(0.7)*(0.3) = 0.42 and ccrccr (q2) = (0.3)2 = 0.09

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