INTERCHANGE WITH SEX CHROMOSOMES 361 



Two geographic races exist, one of which has two pairs of small 

 chromosomes instead of one large pair found in the other. A third 

 race is found in which the males have the single large pair while 

 the females have one large chromosome associated with two small 

 ones (v. Ch. V). Evidently therefore the sex factors are located in 

 this group. Although the third race might have arisen as a cross 

 from the first two it may equally be regarded as an intermediate 

 stage in the change from the one type to the other, for, in the 

 absence of crossing-over, fragmentation in the Y of the large 

 chromosome type will lead to the intermediate type. Only if 

 crossing-over occurs between the sex factors and the point of 

 breakage will the intermediate type yield the small chromosome 

 type. Evidently such crossing-over has taken place. This 

 example therefore provides a diagrammatic illustration of the 

 relationship between crossing-over and the preservation of differences 

 between sex chromosomes {v. Fig. iii). 



In a number of other animals and plants the sex chromosome 

 pair is heterozygous for " fragmentation " of either the X or the Y, 

 giving X-^^X^Y or XY1Y2 systems of segregation (Table 56 and 

 Figs. 115 and 117). 



4. In Humnltis japonicus (Kihara, 1929 b) and H. luptdus (Sinoto, 

 1929) a further structural change has occurred in the extension 

 of the differences in the sex-heterozygote to another pair of 

 chromosomes, while the homozygous sex presumably continues to 

 have simple pairs (Fig. 112). A race of Humulus liipulus has a 

 chain of four instead of one unequal pair and an individual of 

 H. japonicus has been found with a chain of five instead of a chain of 

 three. Each of these modifications can be supposed to be derived 

 from the simpler type by segmental interchange between sex 

 chromosomes and autosomes in the same way as a ring of four is 

 presumed to be derived from two pairs in ordinary interchange 

 heterozygotes. Thus, if the three sex chromosomes of the simpler 

 form of Humulus japonicus are composed of segments (homologues 

 having the same letters) : 



YA-AXB-B 



then segmental interchange between YA and an autosome PQ, 



