LINKAGE 



383 



of the chromosome pairs. Drosophila melanogaster, in which linkage 

 relations have been most fully analysed, has four pairs of chromosomes 

 (Fig. 148) : two large " euchromosome " pairs, one pair of sex-chromosomes, 



FIG. 147. Non-disjunction and its results in Drosophila. The two large circles in 

 first row represent male and female flies producing sperms and eggs respectively. Non- 

 disjunction in the female gives 2 kinds of eggs, with XX and with no sex-chromosomes, 

 instead of the normal single kind with one X. At fertilization there are possible 4 combi- 

 nations rather than 2, as shown in the large circles of second row. Owing to the several 

 ways in which her 3 sex-chromosomes may be distributed at maturation, the female repre- 

 sented by the third circle produces 4 kinds of eggs. When mated to a normal male (below 

 horizontal line) with his 2 kinds of sperms, 8 combinations are possible (last row). Nos. 

 1, 4, and 5 are normal flies and give the usual types of progeny. Nos. 2, 6, and 7, owing 

 to the presence of 3 sex-chromosomes, give exceptional results when bred. Types No. 3 

 and No. 8 do not appear in the cultures, probably because they die very early. The 

 original male has red eyes and the original female white eyes. Red eyes (represented by 

 dots) appear in every fly bearing the .X"-chromosome of the original male, as in Fig. 146. 

 Compare Morgan 1919a, Figs. 93 and 94. (Diagram based on data of Bridges and Morgan.) 



FIG. 148. The chromosomes of Drosophila melanogaster as they appear during mitosis in a 

 female, a male, and a non-disjunctional female. (After Morgan.) 



and one pair of very small "m-chromosomes." The Mendelian charac- 

 ters in Drosophila fall into four linkage groups, and it is noteworthy 

 that one of these groups contains only two known characters. Each 



