184 Heredity and Environment 



ment depends not so much upon the absolute number of chromo- 

 somes in a given cell as upon a complete set of all the different 

 kinds of chromosomes, and when a complete set was not present 

 certain characters failed to develop. By this means he showed 

 that different chromosomes of a set differ in hereditary value, as, 

 for example, the fingers of a hand differ from one another, and 

 that two chromosomes of one kind could not make up for the 

 lack of one of another kind, any more than two thumbs could 

 make up for the loss of a little finger. 



A still more detailed correlation between the presence or absence 

 of a particular chromosome and the presence or absence of parti- 

 cular characters in the developed organism has been described 

 by Bridges (1916). In his study of the pomace fly Drosophila 

 melanogaster, he found that the occasional appearance (i in 1700) 

 of a matroclinous daughter or patroclinous son was due to the fact 

 that the members of the XX pair of chromosomes of the oocyte 

 fail to separate in the reduction division so that both XX's are 

 included in the egg (Fig. 65, C) or both are extruded in the polar 

 body, the eggs being accordingly XX or O; or the two chromo- 

 somes of the XY pair in the spermatocyte fail to separate in the 

 reduction division so that one sperm may have both X and Y, 

 and another lack both of these chromosomes (Fig. 60 a). This 

 phenomenon he calls "non-disjunction" and it results in the pro- 

 duction of matroclinous daughters or patroclinous sons, and in 

 many other irregularities of inheritance which follow precisely 

 the abnormal distribution of these chromosomes. A patroclinous 

 son is the result of the fertilization of an O egg by an X sperm ; 

 such an XO son is sterile whereas the normal XY son is not, thus 

 showing that the chromosome Y has some function though in 

 Drosophila it does not contain any of the genes ; fertilization of an 

 O egg by a Y sperm produces a combination which is non-viable. 

 Fertilization of an XX egg by a Y sperm produces a matroclinous 

 daughter (XXY), whereas fertilization of an XX egg by an X 

 sperm produces a combination (XXX) which is non- viable. 



