6.2.2: Loss of genetic diversity: selection

Selection favours some alleles over others. That is the intention of animal breeding! Obviously this has consequences for the allele frequencies in the next generation. Unlike genetic drift, selection has a systematic and directional force on the change in allele frequencies. The frequency of the favourable allele increases at the expense of the less favourable allele. Consequently, more animals will become homozygous for the favourable allele and genetic diversity decreases.

Exception to this rule are traits where heterozygous animals are favoured. Selection in that case has an increasing effect on genetic diversity. This, for example, is the case with sickle cell anaemia in humans. Sickle cell anaemia is a hereditary disease where red blood cells obtain a sickle shape and lose their flexibility, resulting in various life threatening complications. The disease is caused by a mutation in the haemoglobin gene. People that are homozygous usually die of the complications at fairly early age. In the sub-saharan region the frequency of the allele causing sickle cell anaemia is higher than elsewhere. Important reason for this is that people who are heterozygous for this gene are less affected by malaria infections. Malaria is an important cause of death in that part of the world and carrying a single allele thus has a clear selective advantage. Another example where the heterozygote is favoured is with the Dutch cattle breed the Witrik. The special type of colour marking only occur when they are heterozygous for the gene involved (see figure 3).

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Figure 3: A heterozygous witrik cow with her homozygous witrik calf