Coat coloration in New Mexico Desert mice is controlled by t

Coat coloration in New Mexico Desert mice is controlled by the D locus with two alleles with D completely dominant to d; DD and Dd individuals have dark coloration whereas dd individuals have lighter coloration. You observe a population with 800 individuals with genotype DD, 500 individuals with genotype Dd, and 80 dd individuals

a. What are the genotype and allele frequencies in this population?

b. You determined the survival of these genotypes on dark lava rock habitats during the spring when predator activity was high. The number of individuals is given below. Determine the relative fitness (w) and selection coefficient (s) for each genotype

DD = 400 Dd = 250 dd = 25

c. Calculate the normalized genotype frequencies after selection

d. Calculate the allele frequencies after selection. What happened to the frequency of the D and d allele? Why? Is this form of directional selection dominant or codominance? If you answered dominant, would the change in allele frequency be greater if this were codominance? If yes, why?

Solution

Total individuals = 800+ 500+ 80 = 1380

a)Individuals with DD genotype f(DD)= 800 /1380 = 0.58

Individuals with Dd genotype f(Dd) = 500/1380 =0.36

Individulas with dd genotype f(dd) = 80/1380 = 0.06

allelic frequency f(D) = f(DD) + 1/2 f(Dd)

=0.56 + 0.18

=0.74 ans

allelic frequencyn f(d) = f(dd) + 1/2 f(Dd)

= 0.06+ 0.18

=0.24 ans

b.

Relative fitness( w)

( Divide by the highest fitness i.e 400)

c.Normalised genotype after selection will be :

p² = 0.6/0.6 = 1 ans

2pq = 1- S/2

=1- 0.39/2 =0.8 ans

q² = 1-S

= 1- 0.94 = 0.07 ans.

d.The average fitness W = p^₂ * wDD + 2pq * wDd+ q² *wdd

= 0.6 *1 + 0.37*0.61 + 0.04* 0.07

= 0.6 + 0.22 + 0.003

= 0.82   

Frequency of D after selection will be :

p\' = p2 wDD/W + pq wDd/W

=0.6 * 1/ 0.82 + 0.37 * 0.61 / 0.82

= 0.73 + 0.275 = 0.7

Frequency of d after selection: (1-p’) or q\'= q2 wdd/W + pq wDd/W   

= 0.04 * 0.07/0.82 + 0.37* 0.61/0.82

= 0.3

The allelic frequency of D and d remained almost the same. Since the alleleic frequency of dominant is greater than recessive, it is a directional selection dominant.No, the change in allelic frequency would not be greater if there were codominance as the frequency of both the alleles will be same in case of codominance.