Evolution Connection Biologists believe that the X and Y chromosomes were once a

Question

Evolution Connection Biologists believe that the X and Y chromosomes were once a fully homologous pair. Indeed, it is believed that they evolved from an autosomal pair approximately 300 million years ago. Over the years, many genes have disappeared from the Y causing it to shrink. Additionally, a significant rearrangement of it has led to it being different from the X. Taken together, these keep the X and Y from undergoing crossing over in all but tiny portions at the tips. Hypothesize as to what effect the inability to cross over has done for the X and Y chromosomes, and, by extension, US

Explanation / Answer

Recombination between the X and Y chromosomes proved harmful—it resulted in males without necessary genes formerly found on the Y chromosome, and females with unnecessary or even harmful genes previously only found on the Y chromosome. As a result, genes beneficial to males accumulated near the sex-determining genes, and recombination in this region was suppressed in order to preserve this male specific region. Over time, the Y chromosome changed in such a way as to inhibit the areas around the sex determining genes from recombining at all with the X chromosome. As a result of this process, 95% of the human Y chromosome is unable to recombine. Only the tips of the Y and X chromosomes recombine. The tips of the Y chromosome that could recombine with the X chromosome are referred to as the pseudoautosomal region. The rest of the Y chromosome is passed on to the next generation intact. It is because of this disregard for the rules that the Y chromosome is such a superb tool for investigating recent human evolution. It has also been hypothesised that Y-chromosome is losing genes very fast in the course of evolution, which might someday result in total disapperance of the Y-chromosome.

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