Degeneration Y chromosome

1 degeneration

1.1 high mutation rate
1.2 inefficient selection
1.3 genetic drift

degeneration

by 1 estimate, human y chromosome has lost 1,393 of 1,438 original genes on course of existence, , linear extrapolation of 1,393-gene loss on 300 million years gives rate of genetic loss of 4.6 genes per million years. continued loss of genes @ rate of 4.6 genes per million years result in y chromosome no functional genes – y chromosome lose complete function – within next 10 million years, or half time current age estimate of 160 million years. comparative genomic analysis reveals many mammalian species experiencing similar loss of function in heterozygous sex chromosome. degeneration may fate of non-recombining sex chromosomes, due 3 common evolutionary forces: high mutation rate, inefficient selection, , genetic drift.

however, comparisons of human , chimpanzee y chromosomes (first published in 2005) show human y chromosome has not lost genes since divergence of humans , chimpanzees between 6–7 million years ago, , scientific report in 2012 stated 1 gene had been lost since humans diverged rhesus macaque 25 million years ago. these facts provide direct evidence linear extrapolation model flawed , suggest current human y chromosome either no longer shrinking or shrinking @ slower rate 4.6 genes per million years estimated linear extrapolation model.

high mutation rate

the human y chromosome particularly exposed high mutation rates due environment in housed. y chromosome passed exclusively through sperm, undergo multiple cell divisions during gametogenesis. each cellular division provides further opportunity accumulate base pair mutations. additionally, sperm stored in highly oxidative environment of testis, encourages further mutation. these 2 conditions combined put y chromosome @ greater risk of mutation rest of genome. increased mutation risk y chromosome reported graves factor 4.8. however, original reference obtains number relative mutation rates in male , female germ lines lineage leading humans.

inefficient selection

without ability recombine during meiosis, y chromosome unable expose individual alleles natural selection. deleterious alleles allowed hitchhike beneficial neighbors, propagating maladapted alleles in next generation. conversely, advantageous alleles may selected against if surrounded harmful alleles (background selection). due inability sort through gene content, y chromosome particularly prone accumulation of junk dna. massive accumulations of retrotransposable elements scattered throughout y. random insertion of dna segments disrupts encoded gene sequences , renders them nonfunctional. however, y chromosome has no way of weeding out these jumping genes . without ability isolate alleles, selection cannot act upon them.

a clear, quantitative indication of inefficiency entropy rate of y chromosome. whereas other chromosomes in human genome have entropy rates of 1.5–1.9 bits per nucleotide (compared theoretical maximum of 2 no redundancy), y chromosome s entropy rate 0.84. means y chromosome has lower information content relative overall length; more redundant.

genetic drift

even if adapted y chromosome manages maintain genetic activity avoiding mutation accumulation, there no guarantee passed down next generation. population size of y chromosome inherently limited 1/4 of autosomes: diploid organisms contain 2 copies of autosomal chromosomes while half population contains 1 y chromosome. thus, genetic drift exceptionally strong force acting upon y chromosome. through sheer random assortment, adult male may never pass on y chromosome if has female offspring. thus, although male may have adapted y chromosome free of excessive mutation, may never make in next gene pool. repeat random loss of well-adapted y chromosomes, coupled tendency of y chromosome evolve have more deleterious mutations rather less reasons described above, contributes species-wide degeneration of y chromosomes through muller s ratchet.