84 HEREDITY AND SEX 



is produced — when present in simplex, the result is 

 to produce a male. 



In other words, it is not the sex chromosomes as a 

 whole that determine sex, but only a part of these chro- 

 mosomes. Hence we can understand how sex is deter- 

 mined when an unequal pair of chromosomes is pres- 

 ent, as in lygaeus. The smaller chromosome lacks 

 the sex differential, and probably a certain number of 

 other materials, so that sex-linked inheritance is pos- 

 sible here also. Moreover, in a type like oncopeltus, 

 where the two sex chromosomes are alike in size, we 

 infer that they too differ by the sex differential. If 

 all the other factors are present, as their size suggests, 

 sex-linked inheritance of the same kind would not be 

 expected, but the size difference observable by the 

 microscope is obviously too gross to make any such 

 inference certain. We have come to see that it was a 

 fortunate coincidence only that made possible the dis- 

 covery of sex determination through the sex chromo- 

 somes, because the absence of the sex factor alone in 

 the Y chromosome might have left that chromosome 

 in the male so nearly the same size as the X in the 

 female that their relation to sex might never have been 

 suspected. When, however, one of the sex chromosomes 

 began to lose other materials, it became possible to 

 identify it and discover that sex is dependent upon 

 its distribution. 



THE HEREDITY OF TWO PAIRS OF CHARACTERS 



Mendel observed that his principles of heredity apply 

 not only to pairs of characters taken singly, but to 

 cases where two or more pairs of characters are involved. 



