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 is, in males, the X chromosome has no 

 homologous mate. In such cases half of the 



