/9 2 Heredity and Environment 



Drosophila but they were first studied and are most easily demon- 

 strated in the X chromosome. 



b. Other Cases of Linkage. In addition to characters which 

 are sex-linked other characters may be bound together in hered- 

 ity without being linked with sex. Morgan and his associates have 

 found and studied about four hundred mutations of Drosophila 

 (see Figs. 101-103), which are inherited in four groups, all the 

 characters of each group usually going together. There have been 

 found in the first group 140 different mutant characters, in the 

 second 125, in the third 120 and in the fourth 3. Or eliminating 

 lethal and modifying factors and those of more doubtful location, 

 there remain 188 mutant genes, 50 of which are in the first group, 

 70 in the second, 65 in the third, and 3 in the fourth. Correspond- 

 ing with the number and size of these groups there are four pairs 

 of chromosomes in Drosophila, three of which are large and one is 

 very small (Fig. 65). The sex chromosomes (XX in the female, 

 XY in the male) constitute one of the large pairs and the genes of 

 the characters which are sex-linked are located in these chro- 

 mosomes ; the genes of the second and third groups of characters 

 are in the other large chromosomes, while the fourth group of only 

 three characters have their genes in the very small chromosomes 

 (Fig. 65). If this interpretation is correct, linkage is due to the 



FIG. 65. CHROMOSOMES (DiPLOio NUMBER) OF Drosophila melanogaster. 

 A. Female with 2 X chromosomes; B. Male with i X and I Y; C. Ma- 

 troclinoii'S female (XXY) resulting from non-disjunction of the 2 X 

 chromosomes of the egg. (After Morgan.) 



