The sex chromosomes show a much wider range of viable aneuploidy than do the autosomes, for several reasons. Each diploid somatic cell has only one active X, and most of the genes on any additional X chromosomes are inactivated. The Y chromosome contains very few genes. Mosaicism, with a normal cell line present, is much more common for sex chromosomes than for autosomes. Figure 19.1 summarizes the known nonmosaic numerical sex chromosome abnormalities. In addition to the examples in this figure, the chromosome constitution XYYYY has been found in a few highly abnormal individuals (Noël et al., 1988)..
KeywordsTurner Syndrome Sperm Nucleus Klinefelter Syndrome Gonadal Dysgenesis Pseudoautosomal Region
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