Question

1. Why is it that lethal recessive alleles remain in the human gene pool, rather than just disappearing because of their unfavorable phenotype for homozygous recessives?

2. Scenario: A sudden flood of a large area around the Mississippi River occurs. In some local populations of a species of insects, there is greater buoyancy and survival of some individuals due to having the double recessive genotype of a gene that allowed them to accumulate more lipids to float. A year after the flood, the same and displaced local populations of the insects were reduced in size and all biased in allele frequencies toward greater frequency of the recessive allele compared to the original population’s allelic frequencies. Which of the forces that affect populations seems to have had the strongest effect?

3. Do you think the National Park Service made the correct decision not to permit a genetic rescue of the Isle Royale wolves in 2015, or should new individuals be introduced? Provide reasoning for your answer (as a biologist).

Answer 1

Recessive lethal alleles persist in the population by their presence in the heterozygotes of that genotype. In heterozygote individuals, the lethal allele is present but doesn't show any effect, thereby getting transmitted silently to the next generation. The homozygous recessive individuals die, but the allele persists in the heterozygotes.

In the scenario given above, natural selection seems to have the strongest effect in reducing and biasing the population towards the recessive allele. Here the individuals homozygous for the recessive allele accumulate more lipids in their cells than heterozygous or homozygous dominant individuals, and that helps them to float due to greater buoyancy in the flood. This is clearly a case of altered conditions favoring one particular phenotype over the other and helping it to survive. Hence, natural selection is at play here.

(Here the disaster doesn't wipe out individuals randomly, but is biased towards a specific phenotype.)

The National Park Service did not take the correct decision by not introducing Isle Royal Wolves in 2015, as the remaining population declined drastically after that. If it had intervened when it had the opportunity, then the Isle Royal Wolf population could have increased due to the possibility of outbreeding. Reduced vigour due to continuous inbreeding weakened and reduced the existing population.