THE MENDELIAN METHOD 



X-liiikcd, Y-linkcd and interchangeable. When we examine the 

 sex chromosomes wc see that they have similar parts which pair 

 and cross-over; these are the pairing segments. They also have parts 

 w^hich are dissimilar and which do not pair; these are the differential 

 segments belonging, one to X, and the other to Y, The differential 

 segment of the X is usually larger than that of tlic Y. It usually, 

 as in man, lies at one end, the pairing segment at the other. The 

 distribution of the chiasmata in the pairing segment relative to the 

 centromere and the differential segments then decides whether the 

 first division is reductional for these segments or equational. In the 

 male Drosophila, where crossing-over occurs in the sex chromosomes, 

 although nowhere else, the pairing segment is in the middle of the 

 Y chromosome (separating two differential segments) and at the 

 end of the X. Crossing-over occurs doubly and reciprocally, the 

 second exchange cancelling the first. It therefore keeps the two 

 differential segments of the Y together and does not give any result 

 detectable in ordinary breeding experiments (Fig. 13). 



The same principles of sex determination apply generally not only 

 to animals but also to plants with separate sexes. In the dioecious 

 Campions [Melandrinm dioica) as in Drosophila there are distinct sex 

 chromosomes, the male having XY, and sex-linked genes are found 

 in both X and Y. In different animals there are, however, many 

 differences in detail. In grasshoppers (Orthoptera) and aphids 

 (Hemiptera) there is no Y chromosome. Females have XX, males 

 have one X only. The whole of the X is therefore differential. In 

 some Hemiptera, and in spiders and Ostracoda the X is represented 

 by several chromosomes which move and segregate as a unit in 

 meiosis, the females having twice as many as the males. In Lepi- 

 doptera, birds, and some fishes the female is the heterogametic or 

 XY sex, the male having XX. With these modifications in its 

 chromosome basis, sex-linked inlieritance is correspondingly 

 modified. 



Altogether these observations on mendelian breeding and chromo- 

 some behaviour show us the mechanism of heredity. They show us 

 its common and universal principles. They also tell us something 

 of the material structure and organization of heredity. They show 

 it to be particulate and they show the units, or particles, or genes, 

 into which it is separable to be smaller than the chromosomes. 



52 



