r- 





Chapter 7 



THE GENETIC DISTRIBUTION OF GENES IN 

 THE X AND Y CHROMOSOMES 



It was pointed out in Chapter 5 that there are genes on the 

 X chromosome and rarely on the Y chromosome. Because only 

 one of a pair is present in one sex and because the phenotypes 

 can be correlated with sex, the inheritance of genes on the 

 sex chromosomes can be considered a special problem. 



The inheritance of genes on the sex chromosomes may be 

 considered under three headings: (1) genes on a region of the 

 X chromosome which has no counterpart in the Y chromosomes; 

 (2) genes on a section of the Y chromosome which is not homolo- 

 gous with any section of the X chromosome; and (3) genes 

 located in a chromosomal segment which is homologous in both 

 the X and Y chromosomes. The first type has long been known 

 as "sex linkage." A great many sex-linked genes have been 

 found in animals and a few in plants. The second type may be 

 called 'Y chromosome inheritance." Only a few genes are known 

 in the Y chromosomes of animals. The third type, "incomplete 

 sex linkage," is not common. 



Sex Linkage 



XY and XO Type. As shown in Chapter 6, two alleles such 

 as A and a which are located in a pair of autosomes segregate 

 during gametogenesis or sporogenesis so that half the gametes 

 contain A and the other gametes a. A pair of alleles in the X 

 chromosome of an XX female behaves in exactly the same 

 manner. If this female is heterozygous for A and a, half the eggs 

 produced will have A and the other half will have a. In the 

 male, however, there is only one X chromosome and therefore 

 only one gene of the pair, for even in XY males there is no locus 

 in the Y chromosome corresponding to the locus of a sex-linked 

 gene. If the male has a dominant gene. A, half the gametes will 

 have one X chromosome with the A gene and the other half of 



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