i2 THE POPULAR SCIENCE MONTHLY 



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 (F 2 ) 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 F x female had two kinds of sex chromosomes, that we may call 

 briefly WX and EX. 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-produc- 

 ing spermatozoon, does not carry an X chromosome. When the female- 

 producing sperm fertilizes either kind of egg it brings in the two dom- 

 inant 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 



GR 2870 2 



yw 2373 y W 



GW 36 



YR 34 



X 



w 



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 " cross- 

 ing-over" between the two pairs. The numerals to the left give the F 2 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, 



