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| XXXY syndrome ： ウィキペディア英語版 |
XXXY syndrome
48, XXXY syndrome is a sex chromosome aneuploidy in which males have two extra X chromosomes. Human cells usually contain two sex chromosomes, one from the mother and one from the father. Usually, females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). The appearance of at least one Y chromosome with a properly functioning SRY gene makes a male. Therefore, XXXY only affects males. Males affected with XXXY syndrome have 48 chromosomes instead of the typical 46. This is why XXXY syndrome is sometimes written as 48, XXXY syndrome. It is estimated that XXXY affects one in every 50,000 male births.〔(【引用サイトリンク】title=48,XXYY, 48,XXXY and 49,XXXXY syndromes: not just variants of Klinefelter syndrome )〕
== Causes ==
XXXY syndrome can be caused by the joint non-disjunction in both oogenesis and spermatogenesis or non-disjunction in just oogenesis.〔http://www.mayoclinic.org/diseases-conditions/triple-x-syndrome/basics/causes/con-20033705〕 Non-disjunction which occurs during oogenesis can cause the primary oocyte divides into a secondary oocyte and a polar body. Although the polar body usually dies, both the polar body and secondary oocyte contain genetic information (23 chromosomes). Due to non-disjunction in maternal meiosis I, the secondary oocyte will contain both X homologous chromosome pairs (n+1), leaving no X chromosomes in the polar body. This secondary oocyte will complete regular meiosis II to create a mature ovum with 2 X chromosomes. Another method in which strictly maternal non-disjunction can cause XXXY syndrome is if normal maternal meiosis I occurred but meiosis II had non-disjunction, creating an ovum with 2X chromosomes. In paternal meiosis I, non-disjunction causes XXXY syndrome when the secondary spermatocyte contains both the X and Y homologous chromosome pairs. Normal meiosis II can produce a gamete with 1 X and 1 Y chromosome (n+1). This sperm will then fertilize the mature ovum creating a zygote with 2n+2. Oogenesis which singularly causes XXXY syndrome has non-disjunction in both meiosis I and II. During meiosis II, non-disjunction can cause the mature ovum to contain 3X chromosomes from the division of 4X chromosomes in the secondary oocyte. This ovum will then be fertilized by sperm containing a Y chromosome. Since these sets of occurrences are extremely rare, XXXY syndrome is sporadic.〔http://www.orpha.net/consor/cgi-bin/OC_Exp.php?lng=en&Expert=96263〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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