Question

1. In the White Sands desert of New Mexico, light coloration has evolved in multiple, independent populations of mice, similar to the lizards described by Rosenblum et al. (2010). In some cases, a mutation in the McIR gene prevents binding of a signaling molecule, thus blocking the signal to produce melanin. This type of mutation appears to be recessive. In other cases, increased production of the inhibitor ASP blocks production of melanin. This type of mutation appears to be dominant. In a third type of population, a mutation affecting the melanin pathway creates light coloration in heterozygotes and homozygotes, but also has very detrimental effects on the immune system in individuals that are homozygous. In the table below, Al represents the normal allele and A2 represents the mutation for each case. a. Fill out the table below with fitness values for each genotype. You may use only these three numbers to represent fitness values: 1, 0.8.0.6. For each column in the table, choose fitness values that are consistent with the given scenario. (Note: There may be more than one correct way to do this). b. For the McIR mutation, state the value of the selection coefficients that is consistent with the numbers you have chosen to represent fitness: c. For the melanin pathway/immune system mutation, solve for the equilibrium value of p (the frequency of allele Au given the numbers you have chosen to put in your table. Show your work: Table 1 Fitness Values Melanin ASP mutation pathway/immune system mutation McIR mutation Genotype ΑΙΑ, A,A A2A

Answer 1

	Fitness
Genotype	Mc1R mutation	ASP mutation	Melanin Pathway/immune system mutation
A1A1	0.6	1	0.8
A1A2	0.6	1	1
A2A2	1	0.6	0.6

a.
Mc1R mutation is a recessive mutation. When selection is against a dominant allele A1 or selection is for a recessive allele A2 then the fitness of A1A1 genotype (w11) and A1A2 genotype (w12) is same and is less than the fitness of A2A2 (w22). Thus the respective values of 0.6, 0.6 and 1 are chosen.

                                                            w11= w12< w22

ASP mutation produces a mutated allele which is dominant in nature. When selection is against a recessive allele A2 or selection is for a dominant allele A1, the the fitness of A1A1 genotype (w11) and A1A2 genotype (w12) is same and is more than the fitness of A2A2 (w22). Thus the respective values of 1, 1 and 0.6 are chosen.

w11= w12>w22

Melanin Pathway/ immune system mutation leads to production of mutant light colouration in homozygotes and heterozygotes but has harmful effects on homozygotes. Thus this type of mutation confers an advantage to the heterozygotes. This type of selection is referred to as heterozygous advantage or overdominance. Thus the heterozygous genotype A1A2 has the highest fitness and one of the homozygotes (A1A1 or A2A2) has a higher fitness than the other. Thus the heterozygous genotype has been assigned the highest fitness of 1 followed by 0.8 and 0.6 fitness to A1A1 and A2A2 respectively.

w11< w12>w22
b.

For the Mc1R mutation, the value of selection coefficients of the 3 genotypes are as follows:

Selection Coefficient (s) = 1 – Fitness (w)

Selection Coefficient of A1A1 (s11) = 1 - 0.6 = 0.4

Selection Coefficient of A1A2 (s12) = 1 - 0.6 = 0.4

Selection Coefficient of A2A2 (s22) = 1 - 1 = 0
c.

For the melanin pathway/ immune system mutation, the equilibrium value of p (the frequency of allele A1) is calculated as follows :

s11 = 1-0.8 = 0.2

s12 = 1 – 1 = 0

s22 = 1-0.6 = 0.4

P= s22/s11 + s22

=0.4/0.2+0.4

=0.4/0.6

= 0.67