498 - Heredity and Evolution 



9 



DIPLOID CELLS \Q 



REDUCTION 

 (DIAGRAMMATIC) If 



ZYGOTES 



Fig. 26-21. Distribution of the sex 

 chromosomes in Protenor, a bug 

 that has no Y chromosome. Sex 

 chromosomes, black; autosomes, 

 white. 



sperm cells contain an X chromosome, but 

 the other half contain no sex chromosome 

 (Fig. 26-21). Occasional individuals of Droso- 

 phila (resulting from abnormal maturation 

 divisions) contain two X's and a Y, or even 

 two X's and two Y's, and such individuals 

 are always females. 



Summarizing the evidence, Drosophila pos- 

 sesses a group of genes that determine the 

 female characteristics of the species, and 

 these "female genes" are localized mainly in 

 the X chromosomes. The male character- 

 istics, in contrast, are determined by another 

 group of genes, but the "male genes" have 

 their loci mainly in the autosomes. All indi- 

 viduals are homozygous for the male sex 

 genes so that no difference is found between 

 males and females with reference to the 

 genes for maleness. Thus the primary sex 

 difference is that females, which receive two 

 X chromosomes, possess a double set of "fe- 



male genes," and in such individuals the 

 female characteristics dominate over the male. 

 But male individuals receive only a single set 

 of female determinants, and in these indi- 

 viduals the male characteristics dominate 

 over the female. 



In man and a majority of other organisms, 

 the sex-determining mechanism resembles 

 that of Drosophila. 1 But in some species the 

 female rather than the male displays a dis- 

 similarity of the sex chromosomes, although 

 otherwise the mechanism of sex inheritance 

 is similar to that of Drosophila. 



Sex-Linked Genes and Characters. The Y 

 chromosome of Drosophila and many other 

 species is most unusual. The Y chromosome 

 not only is devoid of sex genes, but it carries 

 scarcely any other genes. In fact, the Y chro- 

 mosome can almost be considered as a 



1 In man, however, the combination XXY yields 

 an abnormal male, as is explained later (p. 507). 



