Why do harmful alleles persist

The lead image is courtesy of Tim Ellis via Flickr. Is evolutionary biology about to prove a two-millennia old metaphysical speculation? Or is metaphysics about to fundamentally change the way we look at…. Put yourself in the mind of a killer whale. A South American legend has it that, centuries ago, a native, suffering from high malarial fever, got lost in the Andes. Thirsty, he drank from a pool…. For example, although there is debate about the issue, some researchers have proposed that the relatively high frequency in European populations of the allele causing cystic fibrosis is a historical holdover from a time when cholera was more rampant in these populations.

It is proposed that carrying the cystic fibrosis allele provided some resistance to cholera and so increased in frequency in earlier European populations. Now that these developed nations are no longer threatened by cholera and the selective environment has changed, natural selection may be slowly weeding the cystic fibrosis allele out of those populations. They may not really reduce fitness Some genetic disorders only exert their effects late in life, after reproduction has taken place.

More Details Learn more about sickle cell anemia or Huntington's disease. The limitations of natural selection. Subscribe to our newsletter. Email Facebook Twitter. For more about Tay-Sachs, follow this link to a Wikipedia article which will open in a new tab.

In the larger American population, the frequency is 1 in , births. What will happen if two people who are carriers for the alleles have children? Each of your parents would have to be a heterozygote Tt. With each child they produced, there would be a one in four chance that this child would inherit two recessive alleles and have the tt genotype.

Half of the children of parents who are heterozygotes will also be heterozygotes Tt. The takeaway message: this harmful allele persists in gene pools because it can hide out in heterozygotes.

There are, of course, harmful dominant alleles. An evolutionary nosology complements traditional medical nosologies and enhances our understanding of the persistence of disease and the meaning of human variation. Abstract Although natural selection might be expected to reduce the incidence and severity of disease, disease persists.