468 THE MAINTENANCE OF SPECIES 



forms a zygote that will produce a female, while the latter sort without 

 the odd chromosome, when uniting with the normal female gamete, 

 produces a zygote that is destined to become a male. Thus, if XX 

 represents the sex chromosomes of the female, and XO those of the 

 male, the result is diagrammatically as follows : 



Germ cells XX XO 



Gametes 

 Zygotes 



In many instances it has been observed that the formula XY, instead 

 of XO, represents the male sex chromosome pair, while the female 

 remains XX. That is, instead of an odd unpaired sex chromosome, 

 there is a mismated pair. The accompanying figure, showing the 

 chromosomes in Drosophila, serves as an example of, such a case. It 

 will be seen in this figure that in the male there are present three pairs 

 of chromosomes in which the mates are alike, but that one chromo- 

 some of the fourth pair is rodlike, while its mate, the F-chromosome, 

 has a bent tip. By substituting Y for the in the preceding diagram, 

 the same explanation for the equality in number of the sexes among 

 the offspring is reached, as in the case of McClung's locusts. In 

 both of these examples it is the number of X-chromosomes present, 



that is, one or two, that deter- 

 V CT mines the sex of the offspring. 



Other variations of this funda- 

 mental idea have been found in 

 the copious investigations which 



nj C have been made on the heredity 



f • of sex, but all agree with Correns' 



^ ^ original interpretation of unlike 



The four pairs of chromosomes in Dro- ^^^ gametes in one parent and 

 sopiiila melanogaster. (Alter Morgan.) . ° . ^ 



like gametes in the other. 



The great majority of plants and animals that have been examined 

 show that the male ordinarily is the sex that produces two kinds of 

 sex-determining gametes. Birds, butterflies, and moths form an 

 exception to this general rule, for in them all the sperm gametes are of 

 one kind, while two kinds of mature eggs, male-producing and female- 

 producing, occur. The result of approximate equality of the sexes in 

 the progeny, however, is the same as in the former instances. 



-^.^ -^..^ 



