Question

In a particular population an investigator measures DNA sequence variation for the X chromosome, the autosomes, the Y-chromosomes, and the mitochondrial genome. For sites free of natural selection and assuming the same mutation rate across chromosomes, what is the expected rank order of neutral heterozygosity these chromosomes? Why? Would you expected similar levels or different levels of heterozygosity for Y-linked vs. mitochondrial DNA for sites free of selection? Explain.

Answer 1

Heterozygosity of chromosomes refers to the extent of variation occuring in a cell due to crossing over event. It is important to note here that the X-chromosome undergoes maximum variation, as the females contain two X-chromosomes and thus have maximum chances of exchanging the genetic material. Followed by it, the Y-chromosome which has less extent of variation since generally the Y-chromosome does not find a compatible chromosome for crosing over. Finally, the mitochondrial chromosome undergoes least or absolutely no crossing over since it is reared and maintained in the mitochondria independently of the nuclear chromosome.

Hence, neutral heterozygosity is maximum for mitochondrial DNA, followed by Y-chromosome and least for X-chromosome.