Introduction to Genetic Analysis 497

Introduction to Genetic Analysis 497 - 44200_15_p481-520...

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496 Chapter 15 Large-Scale Chromosomal Changes XY males. In XXX females, on the other hand, the few transcribed genes are active at only 1.5 times the level that they are in XX females. This lower level of “func- tional aneuploidy” in XXX than in XXY, plus the fact that the active X genes appear to lead to feminization, may explain the feminized phenotype of XXY individu- als. The severity of Turner syndrome (XO) may be due to the deleterious effects of monosomy and to the lower activity of the transcribed genes of the X (compared with XX females). As is usually observed for aneuploids, monosomy for the X chromosome produces a more ab- normal phenotype than does having an extra copy of the same chromosome (triplo-X females or XXY males). Gene dosage is also important in the phenotypes of polyploids. Human polyploid zygotes do arise through various kinds of mistakes in cell division. Most die in utero. Occasionally, triploid babies are born, but none survive. This fact seems to violate the principle that we
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.

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