Question

Severe Combined Immunodeficiency (SCID) is a lethal (soon after birth) recessive genetic defect found primarily in Arabian horses. One study estimated that 5% of Arabian foals were affected by SCID, and that 30% were carriers of the recessive SCID allele.

a. Calculate allele frequencies for the normal and affected alleles at birth.

b. Since SCID is lethal, all affected individuals are eliminated from the population as foals (natural selection). What would be the genotypic frequencies (all of them) in the surviving population after the horses affected by SCID have died?

c. Calculate the allele frequencies in the surviving population.

d. If the surviving population then mates at random with respect to this gene (restoring HW eq), what is the expected frequency of affected horses in the next generation?

Type out and clearly label your work.

Answer 1

a) We have the genotype frequencies, so we have to calcualte the allelic frequencies with them.

q2 = 0.05

2pq = 0.3

Then p2 = 1 - 0.3 - 0.05 = 0.65

Then p = square root 0.65 = 0.8

Then q = square root 0.05 = 0.2

b) We have to eliminate the 0.05, and the rest 0.95 is divided in 0.3 for heterozygous and 0.65 for dominant homozygous. We have to apply a rule of three:

That means 0.316 are heterozygous

That means 0.684 are dominant homozygous

c) If p2 is 0.684, then p = square root of 0.684 = 0.827

And q = 1 - 0.827 = 0.173

d) We have to calculate q2 using the new q value:

q2 = 0.1732 = 0.03, that is 3%