SEGREGATION AT MEWS IS 249 



direct segregation in these males, for as Dzierzon found in 1854, the 

 opposite types of males arise in equal numbers from a hybrid queen 

 (Newell, 1915 ; Whiting and Benkert, 1934 ; Whiting, 1935 b ; 

 cf. Ch. IX). 



8. In every instance in which the sexes are visibly different in 

 their chromosome complements (as they are in most animals and 

 many plants) the segregation of dissimilar chromosomes can be seen 

 in the sex heterozygotes at meiosis, and in some the whole history 

 can be traced, from the random segregation to the development of 

 sexual differences between the dissimilar individuals produced by 

 this segregation (Ch. IX) . In the higher plants the sexual differentia- 

 tion does not express itself in external form until long after meiosis. 

 This is true of the sporozoan Aggregata and has led Dobell (1925) to 

 believe erroneously that the differentiated gametes might be derived 

 from the same haploid cell and so the differentiation might not 

 spring from segregation at meiosis {cf. Naville, 1931 ; Belar, 1926 ; 

 M. Robertson, 1929). 



9. WTiere segregation is occurring in factors carried by chromo- 

 somes represented more than twice (in polysomic and autopolyploid 

 plants) the proportions of offspring bearing the opposite characters 

 occur, not in the simple mendelian proportions but in the more 

 complex proportions to be predicted from the random assortment 

 of the larger number of chromosomes concerned [cf. Haldane, 

 1931 a ; Crane and D., 1932 ; Sansome, 1933 ; Mather, 1935, 

 1936 a). 



The parallelism between chromosome reduction and genetic 

 segregation is therefore complete so far as whole chromosomes are 

 concerned. The specific properties of segregation shown by parts 

 of the same chromosome in the phenomenon of linkage must be 

 considered next. 



3. THE THEORY OF CROSSING-OVER 



(i) Introduction. Crossing-over between chromosomes has been 

 inferred from the structure of bivalents seen at meiosis (Chs. IV, V). 

 It has also been inferred from the proportions of different types of 

 progeny found in breeding experiments. It is now necessary to find 

 out how far the two sets of observations agree, in order that we may 



