. Principles of modern biology. Biology. 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 ge
. Principles of modern biology. Biology. 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 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 a
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