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胎儿从受精开始,就具有来自父母双方的23对染色体,其中一对是性染色体。若性染色体是由XY结合的,发育为男胎;是由XX结合的,发育为女胎。 正常女性及大多数雌性哺乳动物的体细胞内均有两条X染色体。目前认为女性细胞间期核内的两条X染色体,其中一条呈介螺旋、伸展状态,具有活性,参与细胞的代谢活动,也就是间期核内的常染色质部分;另一条呈螺旋化,紧缩状态,无活性,也就是间期核内的异染色质部分。对后一条用特殊染色所显示出的深染致密小体,则叫X性染色质或Barr小体。
The fetus begins with fertilization and has 23 pairs of chromosomes from both parents, one of which is a sex chromosome. If the sex chromosomes are combined by XY, the development of male fetuses; is combined by the XX, the development of female fetuses. Both normal and most female mammals have two X chromosomes in their somatic cells. At present, it is thought that there are two X chromosomes in female interphase nucleus, one of them is a helix, stretched and active, involved in the metabolic activity of cells, which is also the part of the euchromatin in interphase nucleus. Tight state, inactive, that is, interstitial nuclear heterochromatin part. On the latter with a special staining shows the deep-dyed compact body, called X-chromatin or Barr bodies.