CHROMATID SEGREGATION 285 



although the three homologues all differed in this part ; they are 

 therefore derived from the chromatids of one chromosome. But 

 such chromosomes are sometimes dissimilar in other parts ; they 

 must then be the result of crossing-over between chromatids, not 

 between chromosomes. In breeders' language, these " equational 

 exceptions " are said to prove crossing-over in the " four-strand 

 stage," i.e., after division of the chromosomes into chromatids, and 

 never between all four strands at the same point (Bridges and 

 Anderson, 1925 ; Redfield, 1930 ; cf. Bridges, 1916). Such is 

 the method of crossing-over demanded by the chiasmatype 

 interpretation. 



A structurally abnormal type of Drosophila yields similar evidence. 

 Drosophila females with two X chromosomes " attached " to one 

 centromere (v. Fig. 121) and differing in several factors have female 

 progeny with both their X's derived from the same egg-cell of 

 the mother, and these show crossing-over. It is therefore possible 

 to see the constitution of two of the four X chromosomes derived 

 from the four chromatids of one maternal cell and know whether 

 the crossing-over took place before the division into four or after, 

 and whether there is a rule as to the assortment of the four chroma- 

 tids. Thus, if the two chromatids had factors ABCDE and abode, 

 attached at Aa, it was found that flies appeared homozygous 

 for recessive factors. Therefore, again, crossing-over must have 

 taken place after division into chromatids, and between only two 

 of the four, for otherwise crossing-over, being between whole 

 chromosomes, would only yield complementary types, such as 

 ABCde and abcDE. 



Segregation. In the attached-X experiment, it was further 

 found that the proportion of recessives was lower the nearer the 

 factor considered was to the centromere. The segregation of the 

 chromatids is therefore determined at the centromere (Anderson, 

 1925 ; cf. Bridges and Anderson, 1925 ; and Rhoades, 1931). With 

 this system of segregation, genes situated further from the centro- 

 mere should show a higher proportion of homozygosis (and therefore 

 a higher proportion of recessive segregates) approaching, as it were, 

 asymptotically the proportion of one-sixth. The proportions in 

 the X chromosome never reach this value, but agree very closely 



