On the Isle of Man off the coast of Ireland a curious cat re

On the Isle of Man off the coast of Ireland, a curious cat referred to as a manx cat was described in 1845. It had long hind legs and a short tail. Further work showed that the manx characteristics were the result of a dominant M allele at a single locus. An MM zygote would not survive, but Mm genotypes produced the manx characteristics. Imagine a scenario where the M allele arises by mutation on the Isle of Man. Explain how genetic drift could cause the M allele to reach a relatively high frequency (freq(M) > 0.25), even though it has harmful effects on fitness.

Solution

According to the information, the manx cats survive on an island with high frequency of M allele. Although the homozygous dominant individuals with genotype MM do not survive, the frequency of Mm genotype is very high. This can be attributed to high frequency of heterozygotes in this population. A particular explanation for the enhanced survival and hence high frequency of M allele in this population can be provided by the fact that the individuals with Mm genotype might be more suitable to the geographical and environmental conditions of this area.

This can be analogous to the presence of sickle cell trait in humans. In humans, the defective allele in morpholoigical development of red blood cells leads to sickle trait which is lethal when homozygous due to clumping of blood near narrow arteries. However, the heterozygous individuals do survive with proper medications. Interestingly, the individuals heterogygous for sickle trait survive very well and are even resistant to malaria in some regions of the world which are inhabitated by mosquitoes as well. Since the shape of a sickle celled red blood cell is not appropriate for the malaria parasite to survive, such heterozygous patients become resistant to malaria and hence survive. This has gradually lead to increase in frequency of sickle cell allele in this population in heterozygous condition despite the fact that homozygous condition is lethal.

This might hold true for this M allele as well. The mM heterozygotes might be saved from impaired biological function and hence better adapted to the geographical area possibly due to genetic drift. Gradually, this might have lead to increase in frequency of M allele in heterzygous conditions.