|MadSci Network: Genetics|
It is hard to find a good answer for this question. In general, meiosis in a trisomic individual results in "trivalent" formation during meiosis I, with two of the chromosomes paired for part of their length and the third possibly paired with another part of one of the two paired chromosomes. Because the pseudoautosomal region shared between the X and Y is rather small, it is difficult to see how the XYY trivalent would form. This would leave a univalent Y chromosome in XYY male meiosis. There are various consequences of having an unpaired chromosome in meiosis. Often it is lost; sometimes, the chromosome segregates to one pole as a univalent at meiosis I. In this case, there would be a higher incidence of XYY males among the sons of XYY males. XYY males arise from normal fathers by nondisjunction at the second meiotic division, which is rather rare. From an XYY father, YY sperm would arise from segregation of the univalent to one of the poles. It is not easy for a univalent to segregate to one of the poles, because the mechanism for orienting a chromosome pair (a bivalent) at meiosis I requires the opposite orientation of homologous kinetochores. This is not possible with a univalent. The guess in Griffiths et al. is a good guess, if that is what it is; I can't find a good citation detailing the incidence of XYY sons from XYY fathers. Fertility of XYY males is somewhat impaired. There seems to be some mechanism for keeping gametes from meioses with unpaired chromosomes from developing normally. Of course, even if the univalent is always lost, it is possible for an XYY father to have an XYY son by the normal mechanism of nondisjunction at the second meiotic division. With XXY individuals, trivalent formation is more likely, because the two X chromosomes can pair outside of the pseudoautosomal region. From the web: _______ "Individuals with 47,XYY may have hypospadias, small testicles, and undescended testicles (Buyse, 1990), and there may be an association with renal agenesis and renal cystic dysplasia (Rudnik-Schoneborn et al., 1996)." Buyse ME, ed. Chromosome X, Chromosome XYY. In: Birth Defects Encyclopedia. Cambridge, Massachusetts: Blackwell Scientific Publications, 1990:400-401. _______ Here is a good summary on the web: http://www.icomm.ca/geneinfo/xyy.htm _______ Yours, Paul Szauter Mouse Genome Informatics
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