Question

Because the low-fitness allele 'a' is recessive, selection cannot "see" this allele when it exists in heterozygotes. Recall that rare alleles (and we expect highly deleterious alleles to be rare in populations) are most frequently found in carriers. Because of these two facts, it will take more time than one might expect to remove recessive lethal mutations from populations.

Answer 1

Recessive lethal alleles are slowly removed from populations due to their invisibility in heterozygous carriers and low occurrence, making natural selection less effective against them. Dominant lethal alleles are more rapidly removed unless onset is delayed.

Step-by-step explanation:

Selection removing recessive lethal mutations from populations is a slow process. This is due to the fact that recessive alleles are not expressed in heterozygous carriers, rendering them 'invisible' to natural selection. When a deleterious allele is rare, most occurrences are in carriers (heterozygotes), and only a fraction of offspring from two carriers will show the recessive phenotype. Considering that carriers appear normal, they continue to pass on the allele undetected. Additionally, in cases of inbreeding, the likelihood of homozygous offspring increases, a condition known as inbreeding depression. In contrast, dominant lethal mutations are typically removed quickly from the gene pool, as they manifest in both homozygous and heterozygous individuals; however, some may have delayed onset, allowing transmission to the next generation before lethality, such as with Huntington's disease.