8 SEX-LINKED INHERITANCE IN DROSOPHILA. 



the sons due to exchange in the two foregoing crosses is always the same, 

 although they are of contrary classes. Clearly, then, the interchange 

 takes place irrespective of the way in which the factors enter the cross. 

 We call those classes that arise through interchange between the chro- 

 mosomes "cross-over classes" or merely "cross-overs." The phenom- 

 enon of holding together we speak of as linkage. 



By taking a number of factors into consideration at the same time 

 it has been shown that crossing-over involves large pieces of the chromo- 

 somes. The X chromosomes undergo crossing-over in about 60 per 

 cent of the cases, and the crossing-over may occur at any point along 

 the chromosome. When it occurs once, whole ends (or halves even) go 

 over together and the exchange is always equivalent. If crossing-over 

 occurs twice at the same time a middle piece of one chromosome is 

 intercalated between the ends of the other chromosome. This process 

 is called double crossing-over. It occurs not oftener than in about 10 

 per cent of cases for the total length of the X chromosome. Triple 

 crossing-over in the X chromosome is extremely rare and has been 

 observed only about a half dozen times. 



While the genetic evidence forces one to accept crossing-over between 

 the sex chromosomes in the female, that evidence gives no clue as to 

 how such a process is brought about. There are, however, certain 

 facts familiar to the cytologist that furnish a clue as to how such an 

 interchange might take place. When the homologous chromosomes 

 come together at synapsis it has been demonstrated, in some forms at 

 least, that they twist about each other so that one chromosome comes 

 to lie now on the one side now on the other of its partner. If at some 

 points the chromosomes break and the pieces on the same side unite and 

 pass to the same pole of the karyokinetic spindle, the necessary condi- 

 tion for crossing-over will have been fulfilled. 



THE Y CHROMOSOME AND NON-DISJUNCTION. 



Following Wilson's nomenclature, we speak of both X and Y as sex 

 chromosomes. Both the cytological and the genetic evidence shows 

 that when two X chromosomes are present a female is produced, when 

 one, a male. This conclusion leaves the Y chromosome without any 

 observed relation to sex-determination, despite the fact that the Y is 

 normally present in every male and is confined to the male line. The 

 question may be asked, and in fact has been asked, why may not the 

 presence of the Y chromosome determine that a male develop and its 

 absence that a female appear? The only answer that has yet been 

 given, outside of the work on Drosophila, is that since in some insects 

 there is no Y chromosome, there is no need to make such an assumption. 

 But in Drosophila direct proof that Y has no such function is furnished 

 by the evidence discovered by Bridges in the case of non-disjunction. 

 (Bridges, 1913, 1914, 1916, and unpublished results.) 



