CHAPTER XI 

 INHERITANCE OF SEX AND RELATED PHENOMENA 



In the description of the chromosome relations obtaining in the distri- 

 bution of hereditary units, we have had occasion to show how sex in one 

 form, Drosophila a?npelophila, depends upon differences in the chromo- 

 some constitution. In this species three pairs of chromosomes have equal 

 members in both sexes, but the remaining pair in the female consists of 

 two equivalent .XT-chromosomes, in the male of one .XT-chromosome like 

 those in the female paired to an unequal F-chromosome. The distri- 

 bution of sex-linked factors finds a logical explanation in their location 

 in the .XT-chromosomes, and in Drosophila more than fifty sex-linked 

 factors have been studied. But thus far the F-chromosome has not been 

 demonstrated to carry any of those factors which are known to be located 

 in the X-chromosome. When the chromosome relations obtaining in 

 the inheritance of sex in Drosophila are outlined they are found to be 

 as follows: 



Morgan has called this the XY type of sex inheritance. This type 

 of sex inheritance is characterized by the fact that females are homozy- 

 gous for the sex determiners and males are heterozygous for them. 

 Accordingly females produce but one kind of egg with respect to the sex 

 determiners borne by them, but the males produce two kinds of sperm 

 in approximately equal numbers. These two kinds of sperm have been 

 called female-producing and male-producing sperm, because normally 

 when a female-producing sperm fertilizes an egg a female is produced and 

 when a male-producing sperm fertilizes an egg a male is produced. 

 The production of male and female producing sperm in approximately 

 equal numbers and random mating with the egg cells accounts for the 

 approximate equality of the sexes in each generation. 



The XY type of sex inheritance is characteristic of a large number of 

 forms. Apparently all mammals, including man, belong to this type, 

 a number of insects, and the plants Bryonia and Lychnis. The evidence 

 in some cases is based on the results of sex-linked experiments, in some 



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