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The other chromosome pairs, called the autosomesappear to be identical twins; they are superficially indistinguishable. In contrast, the X and Y chromosomes appear to be vastly different from one another.
Why are the sex chromosomes so different? How did they get that way? The Y chromosome is only one-third the size of the X. Although the Y has a partner in X, only the tips of these chromosomes are able to recombine. Thus, most of the Y chromosome is inherited from father to son in a pattern resembling asexual, not sexual, reproduction.
No recombination means no reassortment, so deleterious mutations have no opportunity to be independently selected against. The Y chromosome therefore tends to accumulate changes and deletions faster than the X. Degradation doesn't occur in X chromosomes because during female meiosisthe X has the other X as a full partner in recombination.
Clues of how the Y chromosome evolved can be found by comparing the genes and the sequences of X and Y chromosomes as well as homologous genes of different species. One method scientists use to estimate evolutionary time is observing how homologous genes have become different over time in different species.
All DNA sequences accumulate random mutations over time, so species that are distant relatives should have more different sequences than close relatives because they have been evolving separately for a longer time. Once recombination stopped between portions of X and Y, genes located on those parts started to evolve separately as homologs.
Apparently, this happened in stages, so some X-Y gene pairs are more related than others, meaning they stopped recombining more recently.
Also, chunks of genes stopped recombining, and by mapping their positions on the chromosome, one can guess that an event, like an inversionmay have taken place.
Travel back in time, when human ancestors were reptile-like forms, and peer into the processes that shaped the X and Y chromosomes. Introduction How did the human Y chromosome become so small relative to its X counterpart?
This animation depicts the million-year odyssey of the sex chromosomes that began when the proto X and Y were an identical pair. Over time, structural changes in the Y chromosome resulted in its current form, which is specialized to trigger male development. The evolutionary timescale is represented by positioning the chromosomal remodeling events along an abbreviated vertebrate cladogram, a chart of evolutionary relationships.
Sex chromosomes originated as autosomes The sex chromosomes began as an ordinary pair of autosomes. Inversions restrict recombination between the X and the Y chromosome Inversionswhich are internal recombination events, caused a rearrangement of genes on the Y chromosome.
These rearrangements meant that large portions of the X and Y chromosome no longer recombined, which made the Y chromosome susceptible to deletionsand it decreased in size.
Comparison of sex-chromosome recombination in males and females After our lineage diverged from the ancestors of the monotremes, such as the duck-billed platypus, another inversion further scrambled the genes on the proto Y. In males, only the tips of the Y chromosome were left able to recombine with homologous genes on the X chromosome.Link x Amiibo.
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TheEducationalNerd. Presenting the Queen of the Fairies - Titania Erza Scarlet. More About Evolution of The Y Chromosome. Background. The human X and Y chromosomes are a unique pair.
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