HEREDITY 275 



parents have heterozygous brown eyes, there is one chance out of 

 four for blue-eyed children. 



Sex Inheritance. — It has been pointed out in a preceding 

 chapter (XI) that there is a correlation between the sex of the 

 individual and the kind and number of sex chromosomes present. 

 In Anasa tristis, the female condition is accompanied by the 

 presence of 2 X chromosomes in the body cells, while the cells of 

 the male possess only 1 X chromosome. It was also brought out 

 that in other animals where the number of chromosomes in the 

 two sexes is alike there are differences in form, size, or behavior of 

 certain members of the chromosome complexes of the two sexes. 

 It can therefore be accepted that under normal conditions there 

 is a constant relation between the determination of sex and the 

 kind and number of sex chromosomes present in the fertilized 

 egg. Sex may be regarded as an inherited condition whose 

 genes are located, in part at least, in the sex chromosomes. In 

 dichogamous animals the sex of the individual may be male at 

 one time and female at another. 



In the fruit fly, Drosophila, 2 X chromosomes in an individual 

 do not necessarily establish femaleness if there are irregularities 

 in the number of autosomes. In Drosophila the zygotic or 

 diploid complex is normally composed of 4 pairs of chromosomes 

 consisting of a pair of sex chromosomes, known as group I, and 

 3 pairs of autosomes, known as groups II, III, and IV, in order of 

 decreasing size. The difference between the male and female 

 diploid groups is that group I in the male is composed of an X 

 and a Y chromosome, while in the female it consists of 2 X 

 chromosomes. The autosome groups II, III, and IV are the 

 same in both sexes. Therefore under normal conditions in the 

 female the ratio of X chromosomes to sets of autosomes is 2:2 

 (2 X chromosomes to 2 of each of autosomes II, III, and IV). 

 In the male the normal ratio is 1 : 2. A fly having a ratio of 3 : 3 

 is still a female; but one having a ratio of 2:3 is a sex intergrade, 

 combining qualities of both sexes. 



From such facts it would appear that the genes for sex are 

 present in autosomes as well as in the sex chromosomes and that 

 maleness or femaleness depends upon the proportions of these two 

 kinds of chromosomes. Since, however, under normal conditions 

 the fertilized egg contains a double set of autosomes, the sex of 

 the individual developing from such an egg is determined by 



