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 ampelophila, 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 X-chromosomes, in the male of one X-chromosome like 
those in the female paired to an unequal Y-chromosome. The distri- 
bution of sex-linked factors finds a logical explanation in their location 
in the X-chromosomes, and in Drosophila more than fifty sex-linked 
factors have been studied. But thus far the Y-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: 
XX x GY. 
| ay =e 
X X Y 
ee ee 
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 js 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 
196 
Digitized by Microsoft® 
