Page:Popular Science Monthly Volume 84.djvu/16

12 to a gray (wild) female with red eyes, the daughters are gray with red eyes, and the sons also (Fig. 4). The explanation of this result is shown in the diagram in which the sex chromosomes are represented by X's which are marked like the characters they stand for. The daughter gets a "red-gray" chromosome from the mother and a "white-yellow" chromosome from the father. Since red and gray dominate the color is determined by these factors. The son gets his single sex chromosome from his mother which carries the factors for gray body color and red eyes.

When the hybrid (F1) flies are inbred they produce one kind of female and four kinds of males as shown in the next figure. In order to understand how these classes arise let us follow the history of the sex chromosomes.

The F1 female had two kinds of sex chromosomes, that we may call briefly WX and RX. Either may pass out into the polar body leaving the other chromosome in the egg. Consequently there are two kinds of eggs. The F x male has only one sex chromosome GRX which goes into the female-producing spermatozoon. The other, the male-producing spermatozoon, does not carry an X chromosome. When the female producing sperm fertilizes either kind of egg it brings in the two dominant factors GR; hence all the females are gray in body color and have red eyes. Since the male-producing sperm does not bring into the egg

 Fig. 5. The two upper lines represent the two sex chromosomes of the female, showing their original composition, viz., GR and YW. The lower line shows "crossing-over" between the two pairs. The numerals to the left give the F2 males, whose composition is the same as that of the chromosome that stands next to them. The males receive one or the other of these chromosomes from their mother.

any X chromosome, the sex-linked factors in that egg are those of the egg itself. Since without interchange between the sex chromosomes there are two kinds of eggs there will be two kinds of males, namely, gray-white and yellow-red.

But the experiment shows that there are four kinds of males. Their origin can be explained if we assume that in some of the eggs the sex chromosomes prior to the extrusion of the polar bodies have crossed,