80 AN INTRODUCTION TO MODERN GENETICS 



Factors in the Y will remain associated with the heterogametic sex, 

 descending simply in the male line in male heterogamety, and may 

 simulate sex limited inheritance unless abnormal organisms such as 

 XO males or XX Y females can be found which allow the Y chromo- 

 some factors to be dissociated from the sex-controlling mechanism. 

 Examples of Y chromosome genes are comparatively rare, since the Y 

 usually consists nearly entirely of inert heterochromatin. They are 

 found chiefly in organisms in which the genetic sex-determining 

 mechanism is not highly developed, e.g. in some fish (Lebistes),^ and in 

 these forms may show a fairly high percentage of crossing-over into 

 the X. In organisms with more highly evolved sex chromosomes, 



X>^X^? by XWy^ XbbXbb by XbbyBB^ 



white eyed I red eyed bobbed I non-bobbed 



X^XWc 



^9 X^/c^ XbbXbb$ XbbyBB^ 



red eyed white eyed bobbed non-bobbed 



Fig. 3^. Sex Linkage in the X and Y. — In cases of female homogamety, a 

 daughter must get one of her X chromosomes from her father, a son must get 

 his single one from his mother; thus if a recessive female is crossed to a 

 dominant male, the sons are like their mother and the daughters like their 

 father (on left, X^ = recessive white factor in X, etc.). Factors in the Y are 

 handed on from father to son (on right, bb = bobbed). 



between which crossing-over is more completely suppressed, few Y 

 chromosome factors are known. In Drosophila melanogaster the bobbed 

 locus, which causes short bristles, is the best known. There are also 

 two regions of the Y which are both necessary for fertility in males; 

 they probably contain factor complexes. 



Crossing-over between the X and Y gives rise to partial sex-linkage, 

 a phenomenon which is best seen in fish. It has also been shown to 

 occur in Drosophila for the locus of bobbed, which may cross over from 

 the Y into the X if a chiasma occurs farther than usual away from the 

 centromere. Partial sex-linkage has recently been detected in man 

 (p. 332). 



B. NON-DISJUNCTION 



Two chromosomes associated at meiotic metaphase may fail to 

 separate ("disjoin") in the anaphase and may therefore pass to the 

 same pole, and, dividing equationally in the second division, they may 

 ^ Cf. Winge 1923, 1927, 193 1. 



