Question

A large population of fish live in a lake. The color of their scales is determined by two different alleles of the S gene, S1 and S2. Homozygous S1 fish have yellow scales, S2 homozygotes have blue scales, and heterozygotes are green. Scientists catch 100 fish at random and record their color. Among those 100 fish are 30 yellow, 50 blue, and 20 green ones. After the original sample is analyzed, an asteroid landed in the lake and killed all the fish that happened to be in one location. When 100 fish were sampled again, 15 were yellow, 75 were blue, and 10 were green.

Part 1: What is the frequency of the S¹ allele in the population before the asteroid hit?

• 0.15

• 0.40

• 0.50

• 0.60

• 0.80

Part 2: What is the frequency of the S¹ allele in the fish population after the asteroid hit?

• 0.15

• 0.20

• 0.25

• 0.40

• 0.80

Part 3: By what mechanism did allele frequencies change due to the asteroid?

• Allele frequencies did not change because of the asteroid.

• The asteroid caused mutations in the fish population.

• The asteroid changed the environment, so fish with allele S² had increased fitness.

• The asteroid killed fish because of their location, so the allele frequency changed by chance.

Answer 1

Before asteroid hit:

Genotype frequency ofS1S1=p2=30/100=0.30

Genotype freq of S1S2=2pq=20/1000=0.20

Genotype freq of S2S2=q2=50/100=0.50

S1 allele frequency =p=S1S1freq+(1/2)S1S2 freq

S1 allele frequency =0.3+(1/2)x0.2=0.40

Part 1: S1 allele freq before asteroid hit=0.40

After asteroid hit:

S1S1 genotype freq=p'2=15/100=0.15

S1S2 genotype freq=2p'q'=10/100=0.1

S2S2 genotype freq=q'2=75/100=0.75

S1 allele freq=S1S1 freq+(1/2)S1S2 freq

S1 allele frequency after asteroid hit=0.15+(1/2)x0.1=0.20

Ans part 2: S1 allele freq after asteroid =0.20

Ans)part3:

The asteroid killed fish because of their location, so the allele frequency changed by chance.