LI \ KAGE 





of the chromosome pairs. Drosophila melanogaster, in which Linkage 

 relations have been most fully analysed, has lour pairs of chromoson 

 (Fig. 148): two large "euchromosome" pairs, one pairofsex-chromosom< 



DROSOPHILA 



MOM Pi ' UM( ; 



f 



HINDS 



toos 



/* 



0"~" ^ 



T ) ( *tY )( ,Xt j f tt j 



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 Bex-chromoson 

 instead of the normal single kind with one X. At fertilization there are possible 1 combi- 

 nations rather than 2, as shown in the large circles of second row. I ►wing to th<' wveral 

 ways in which her 3 sex-chromosomes may be distributed at maturation, tin 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 (las! 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 N 

 and No. 8 do not appear in the cultures, probably because they die very early. 1 la- 

 original male has red eyes and the original female white eyes. Red eyes "•'! by 

 dots) appear in every fly bearing the X-chromosome of the original male as in 1 

 Compare Morgan 1919a' Figs. 93 and 94. (Diagram based on <lntn of Bridges mid M 



A A ^| 



A 



? 



J* 



? 



Fig. 148.- 



The chromosomes of Drosophila >n< lanogasU r :i- they appear during mitosis in a 

 female, a male, and a non-disjunctional female. I I 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 



