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Jun
21
    
Posted (crates) in biology, Evolution, Genetics on June-21-2007

Sometimes in an effort to understand something, I’ll read about it and make an outline for myself which helps me to grasp the main ideas. I’ve been reading about the Y chromosome, and about some of the implications of recent research about this gender determining chromosome.

You may already know that sex in humans is determined by the Y chromosome. Females have two of the X sex chromosomes, whereas males have XY. These two chromosomes, although probably very similiar originally, diverged more and more after the male determining gene became ensconced on the Y chromosome probably over 300 million years ago. Below is an outline based on information presented by a paper by Willard in Nature and a medley of other sources.

  • The two human sex chromosomes, X and Y, originated about 300 hundred million years ago from the same ancestral autosome — a non-sex chromosome — during the evolution of sex determination
  • They gradually diverged in their base sequence.
  • At either end of the Y chromosome, however, there are still regions that are similar to the X chromosome with crossing over occurring between these areas.
  • 95% of the Y chromosome is specific to this chromosome (about 23 million base pairs) which is referred to as the Male Specific Region of the Y, or MSY and involves no crossing over with the X chromosome. This includes the gene that determines maleness (SRY) and any other genes beneficial to males.
  • 10-15% of the MSY include sequences (the X transposed) that moved from the X chromosome only in the past few million years, and are still 99% identical to the corresponding sections on the X chromosome.
  • Another 20% of the MSY consists of a class of base sequences that appear to be more distantly related to the X chromosome and probably are pieces of ancient autosomes from which both the X and Y chromosomes evolved. These are the X-degenerate sequences. The rest of the bases are specific to the Y chromosome, and tend to be repetitive palindromes—reading the same on both DNA strands (the ampliconic sequences).
  • Insights have been made into the evolutionary strategies that the Y chromosome has made in order to survive.
    • Unlike the sequences on the ends, the MSY section of the Y chromosome does not exchange segments with the X chromosome. This can be seen to be necessary to prevent complications. If it did occur, then males would be missing both essential Y chromosomal genes and X chromosomal genes, and females could have extra genes on their X chromosomes which were obtained from the Y chromosome. Females could also end up missing essential X chromosomal genes.
    • Selection appears to have occurred on the testis specific genes that may enhance fertility.
    • Most of these genes occur in the palindromes mentioned above.
    • However, as crossing over between the Y and X chromosomes ceased, the danger of accumulating mutations that would inactivate the Y chromosome genes would tend to increase.
    • These mutations, no longer eliminated through recombination, would eventually cause the demise of the entire chromosome.
    • Apparently to prevent this from happening, the sequence on one arm of the palindrome can alter or convert the sequence on the other arm—crossing over (recombination) between the two DNA strands. Apparently this has been happening in the larger primates at least since the line leading to humans branched off (five million plus years).
    • It was calculated that as many as 600 base pairs in each new born male must be converted in this way.
  • This led to the conclusion that this gene conversion may be more common in other areas of the genome, especially in palindromic and other duplicated areas.
  • This supports a dynamic view of genome change in which not only do mutations occur (as much as 100-200 base pairs per person), but that thousands of gene conversions occur also. Skeletsky et al. sequenced the Y chromosome.
  • Theoretically if the Y chromosome was eventually reduced down to just the gene causing maleness (SRY), then the gene could attach to either an X chromosome or another non-sex chromosome (autosome) which would create a new Y chromosome. Another possibility could involve a piece of an autosome attaching itself to the sex chromosomes. Both these things have happened in other organisms.

Out of curiosity I took a test on bias. There were several tests on the subject, and I took the one dealing with gender bias in the sciences. My results were: “Slight association of males with science and females with liberal arts compared to females with science and males with liberal arts.” In my opinion the way the test was given automatically predisposes the results towards this sort of bias.

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