CHROMOSOMES AND GENES 55 



two X's and the Y, because the X's may conjugate, leav- 

 ing the Y free to move to either pole, or one X and the Y 

 may mate, leaving a free X. Possibly all three may come 

 together, and then separate so that two go to one pole of 

 the maturation spindle and one to the opposite pole. The 

 results are practically the same in either case. Four kinds 

 of eggs are expected, as shown in the diagram (Fig. 35). 



In order to follow the genetic changes it is necessary 

 that the X-chromosomes of the female or of the male carry 

 one or more recessive genes. For instance, if the two X's 

 in the female carry each the gene for white eyes, and the 

 X in the male carries the allelomorphic gene for red eyes, 

 and if the former are indicated by open (white) X's and 

 the latter by a black X (Fig. 35), the combinations that 

 result are those indicated in the diagram (Fig. 35). Eight 

 kinds of individuals are expected, one of which (YY), not 

 containing even one X-chromosome, is expected to die. In 

 fact, this individual does not appear. Two of these indi- 

 viduals, viz., No. 4 and No. 7, never appear when an ordi- 

 nary white-eyed (XX) female is fertilized by a red-eyed 

 male. Their presence here, however, is in accord with the 

 expectation from an XXY white-eyed female. They have 

 been tested by genetic evidence and found to correspond 

 to the formula here given them. Furthermore, the white- 

 eyed XXY female has been also shown, by cytological 

 examination, to have two X's and a Y in her cells. 



There is one additional kind of female expected that 

 has three X-chromosomes. The diagram indicates that 

 she dies, and this happens in the great majority of cases ; 

 but rarely one comes through. She has certain peculiari- 

 ties by which she can be easily identified. She is sluggish, 

 her wings are short and often irregular (Fig. 36) and she 

 is sterile. A microscopic examination of her cells has 

 shown that she contains three X-chromosomes. 



This evidence points to the correctness of the theory 



