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The X and Y Chromosomes

The X and Y Chromosomes. X: very similar to autosomes; many genes, mostly un-related to sex. X is more evolutionarily conserved than autosomes: genes on X in one species usually on X in others, although often rearranged by inversions.

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The X and Y Chromosomes

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  1. The X and Y Chromosomes

  2. X: very similar to autosomes; many genes, mostly un-related to sex. X is more evolutionarily conserved than autosomes: genes on X in one species usually on X in others, although often rearranged by inversions

  3. X-inactivation: Lyon hypothesis: placental mammals randomly inactivate all but 1 X at the 200-400 cell embryo stage (blastocyst). The inactivated X's become Barr bodies: late-replicating condensed chromatin sitting on the nuclear membrane (heterochromatin). Seen in XXY, XXX, etc.

  4. Mechanism of inactivation: XIC (X inactivation center) first seen as a spot where the inactive X first folds after mitosis (going back to blob state), in Xq13.

  5. XIST is a gene in the XIC region which is expressed only from the INACTIVE X. It has 8 exons, but no open reading frames (structural RNA).

  6. PAR: pseudo-autosomal regions on tips of X and Y: homologous, contain some genes. PAR1 has a required cross over for successful sperm development.

  7. --X and Y pair in meiosis and have crossing over in PARs

  8. Y-chromosome --mostly inert, very few genes, mostly repeat sequence DNA (high and middle).

  9. Sex Chromosome Aneuploidies

  10. X chromosome polyploidy 47, XXY (Klinefelter’s syndrome) extra X chromosomes predispose to azoo and female phenotype development (micropenis, cryptorchidism, etc.), mental retardation, disproportionate growth of the legs, and other somatic anomalies, with estimated incidence of 1:500 in newborns.

  11. Klinefelter patients may occasionally demonstrate motile sperms in the ejaculate which is speculated to be due to a mosaic 46, XY cell line.

  12. Y chromosome polyploidy: Estimated incidence is 1:750 newborns, may be fertile with frequent miscarriage of their wives, perinatal death and chromosomal anomalies of their children.

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