. Principles of modern biology. Biology. REDUCTION (DIAGRAMMATIC) ^5^ Fig. 26-20. Diagram showing the distribution of the sex chromosomes of Drosophila melanogaster. Sex chromosomes, black; autosomes, white. ZYGOTES ample, chromosome pair number 1 consists of two straight rods, each the apparent dupli- cate of the other; but in the male Drosophila, one member of pair I is bent, and is plainly longer than the other (Fig. 26-20). In Dro- sophila the sex-differentiating genes are car- ried in the chromosomes of pair I. Accord- ingly these chromosomes are called the sex chromosomes, in contrast to
. Principles of modern biology. Biology. REDUCTION (DIAGRAMMATIC) ^5^ Fig. 26-20. Diagram showing the distribution of the sex chromosomes of Drosophila melanogaster. Sex chromosomes, black; autosomes, white. ZYGOTES ample, chromosome pair number 1 consists of two straight rods, each the apparent dupli- cate of the other; but in the male Drosophila, one member of pair I is bent, and is plainly longer than the other (Fig. 26-20). In Dro- sophila the sex-differentiating genes are car- ried in the chromosomes of pair I. Accord- ingly these chromosomes are called the sex chromosomes, in contrast to the other auto- somes, which ordinarily are neutral in their effect upon sexual development. Moreover, the sex chromosomes of Drosophila are of two types: the straight type is called the X chromosome, and the bent type, the Y chro- mosome. The sex chromosomes, like the autosomes, are transmitted to the offspring in the usual fashion, as may be seen in Figure 26-20. But due to the dissimilarity between the X and the Y, equal numbers of males and females are conceived in each new generation of the species. As to their bearing on the sex of the. GAMETES offspring, all the eggs of a female are of one kind, in that all carry an X chromosome. But a male produces two kinds of sperm— half X-bearing and half Y-bearing. When one of the X-bearing sperm succeeds in fer- tilizing an egg, the offspring will be female; and the Y-bearing sperm can only give rise to male progeny. These microscopically visible facts suggest that each X chromosome carries a recessive gene for femaleness, and that each Y chro- mosome carries a dominant gene for male- ness; but this is not true. Actually the Y chromosome has no direct effect in deter- mining sex. By certain rare abnormalities in the maturation divisions, individuals of Drosophila are produced that have one X and no Y chromosome; and these individuals are always males. In fact some animals trans- mit the sex differences in just this fashion; that
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