CHROMOSOMES AND MENDELIAN HEREDITY 297 



that of sex (McClung). In certain insects it was observed that one pair 

 of chromosomes was markedly different from the others in appearance and 

 behavior; moreover, the two members were unUke in size, so that they 

 could be followed through successive generations. Furthermore, males 

 and females were observed to differ with respect to this chromosome pair, 

 females commonly having two large ones {XX) while the males had one 

 large and one small {XY). In some species the Y was absent altogether, 

 so that the two sexes were characterized by different chromosome num- 

 bers. The logical conclusion that this chromosome pair exerts a special 

 influence upon the sex of the organism has since been borne out in a large 

 number of unisexual animals and plants. In typical cases this "sex- 

 chromosome" mechanism tends to produce equal numbers of males and 

 females in each generation. A number of characters other than sex were 

 seen to be linked with sex in inheritance, so that their genes were assigned 

 to the sex-chromosomes. Such genes are said to be "sex-linked. " These 

 topics will be discussed further in Chapter XXIII. 



The studies of Morgan and his associates on Drosophila melanogaster 

 have shown that pair I, which is heteromorphic in the male, has a special 

 influence on sex determination and carries a considerable group of genes 

 for sex-linked characters. Pair IV, the smallest of the group, carries 

 the very small linkage group, as was shown particularly well by occasional 

 flies which had one too many or too few of these small chromosomes. The 

 two large linkage groups were accordingly assigned to the two large 

 chromosome pairs, and it has since been demonstrated that the slightly 

 longer pair carries the genes of the "third" linkage group (Dobzhansky, 

 1929a6). 



In Zea Mays the 10 chromosomes of the monoploid set (and hence the 

 10 pairs) can all be distinguished on the basis of their size and structure 

 (McChntock, 19296, 1932c). It has now been possible to identify each 

 linkage group worked out by Emerson and his associates with one of 

 these 10 chromosomes (Fig. 170). This has been accomplished largely 

 through a study of occasional "trisomic" plants. In such plants one of 

 the members of the set is present in triplicate in the somatic cells while the 

 nine others are in duplicate as usual ; hence at the time of meiosis some 

 spores, and therefore gametes, receive one member of the "trisome" 

 while others receive two. From this it follows that any characters due to 

 genes in the triplicate chromosome should appear in abnormal proportions 

 in successive generations, while other characters should at the same time 

 appear in normal Mendelian ratios. The method is, therefore, to deter- 

 mine genetically which characters give "trisomic ratios" when the plants 

 are bred, and to observe cytologically which chromosome of the .set is 

 present in triplicate in the somatic cells and in duplicate in some of the 

 microspores. In this way it was found that in a strain of maize with an 

 extra chromosome, number X (the smallest of the set), an aleurone color 



