Introduction to Genetic Analysis 494

Introduction to Genetic Analysis 494 - 493 15.1 Changes in...

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Unformatted text preview: 493 15.1 Changes in chromosome number drome are males with lanky builds and a mildly im- paired IQ and are sterile (Figure 15-16). Another abnor- mal combination, XYY, has a controversial history. At- tempts have been made to link the XYY condition with a predisposition toward violence. However, it is now clear that an XYY condition in no way guarantees such behavior. Males with XYY are usually fertile. Meioses show normal pairing of the X with one of the Ys; the other Y does not pair and is not transmitted to gametes. Therefore the gametes contain either X or Y, never YY or XY. Triplo-X trisomics (XXX) are phenotypically normal and fertile females. Meiosis shows pairing of only two X chromosomes; the third does not pair. Hence eggs bear only one X and, as in the case of XYY individuals, the condition is not passed on to progeny. Of human trisomies, the most familiar type is Down syndrome (Figure 15-17), which we discussed briefly at the beginning of the chapter. Down syndrome occurs atthe beginning of the chapter....
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