Why do deleterious alleles tend to be recessive?

Why do deleterious alleles tend to be recessive?

• A. Dominant deleterious alleles are easily identified and removed, allowing recessive alleles to persist.
• B. Recessive alleles are always eliminated by selection.
• C. Dominant deleterious alleles are advantageous in carriers.
• D. Deleterious alleles are never found in heterozygotes.

Answer: (A)

Deleterious alleles tend to be recessive because natural selection acts on visible traits, not hidden genotypes. If the harmful effect only shows up when two copies are present, individuals carrying just one copy (heterozygotes) look normal and aren’t strongly penalized by selection. This lets the allele persist in the population at low frequency as a carrier state. If the deleterious allele were dominant, anyone with even one copy would show the harmful trait, making it obvious to selection and quickly removed from the population. That’s why many harmful mutations stay in the gene pool as recessives: they’re hidden in carriers and only harmful when homozygous, while dominant deleterious alleles are efficiently purged. For example, in a recessive disease like cystic fibrosis, carriers are healthy, so the allele can remain in populations at a low frequency; affected individuals are rarer because two copies are required. Inbreeding, which increases homozygosity, can then reveal these recessive deleterious alleles and increase their deleterious effects.

Deleterious alleles tend to be recessive because natural selection acts on visible traits, not hidden genotypes. If the harmful effect only shows up when two copies are present, individuals carrying just one copy (heterozygotes) look normal and aren’t strongly penalized by selection. This lets the allele persist in the population at low frequency as a carrier state. If the deleterious allele were dominant, anyone with even one copy would show the harmful trait, making it obvious to selection and quickly removed from the population. That’s why many harmful mutations stay in the gene pool as recessives: they’re hidden in carriers and only harmful when homozygous, while dominant deleterious alleles are efficiently purged. For example, in a recessive disease like cystic fibrosis, carriers are healthy, so the allele can remain in populations at a low frequency; affected individuals are rarer because two copies are required. Inbreeding, which increases homozygosity, can then reveal these recessive deleterious alleles and increase their deleterious effects.