588 CALVIN B. BRIDGES 



In the exceptional case that I have found, where the mating 

 was hke that just described, about 5 per cent of the daughters 

 are like the mother and 5 per cent of the sons are like the 

 father. 



This anomolous result may be explained, if, in such mothers, 

 a certain per cent of maturations are of a type characterized by 

 non-disjunction, ie., eggs are formed which contain two sex chromo- 

 somes instead of the normal one, and other eggs corresponding to 

 them contain no sex chromosome. If in one of the two maturation 

 divisions of the egg, the two sex chromosomes that usually pass 

 to opposite poles, one going out in the polar body and the other 

 remaining in the egg, should sometimes not disjoin from each 

 other, but should lag upon the spindle so that both pass into the 

 polar body or the egg, then there would result eggs with two 

 chromosomes and other eggs with no sex chromosome. The eggs 

 of such a female will then be of three classes, namely, a large 

 normal class with one sex chromosome, and two small equal 

 classes containing respectively, two and no sex chromosomes. 



If sex linked characters are determined by factors carried in 

 definite loci in the sex chromosome, then we have a way of trac- 

 ing the maternal and paternal chromosomes. For example, all 

 the X-chromosomes of a white female, which produces the three 

 kinds of eggs above, are white bearing. Any X-chromosome 

 of the wild male, to which she is mated, is red bearing. 



The results of this cross are summarized in the diagram be- 

 low,2 and are as follows: 



90 5 5 



Per cent Per cent Per cent 



Eggs of white female w 

 Gametes of red male W 



Per cent Per cent 



(1) wW red 9 90l (2) w — white c^ 90 



(4) WWW red 9 5/ (3) VVW— white 9 5 



(5) — W red cf 5 (6) • 5 



(1) The one-X egg fertilized by the one-X sperm will give a 

 female (X X), red, heterozygous for white. (2) The one-X egg 



2 In the diagram the w indicates the sex chromosome which bears white, and 

 the VV the red bearing chromosome. The dash represents the no-X gamete.. 



