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THE AM ERIC AX XA TURALIST [Vol. XLVII1 



portance in this connection. "When a male heterozygous for S 

 and for Y, one dominant having been derived from each parent 

 (SysY), was mated to a doubly recessive (sysy) female, there 

 were produced 63 Sy and 65 sY — no cross-overs. A female 

 heterozygous also for 8 and for Y, but having them "coupled" 

 (SYsy), was mated to a male sysy, and produced 215 SY and 

 188 sy — again no cross-overs. Yet that crossing over may occur 

 between these two pairs of genes is shown by the fact that the 

 "coupling" F 2 results indicated a gametic ratio of about 

 7:1:1:7. We are, therefore, still left in the dark as to whether 



that the strength of linkage in this case is not always the same — 

 a point of great interest and importance. Similar cases have been 

 reported by Banr 3 in Antirrhinum, by Punnett 1 in the sweet pea, 

 and by the writer 5 in Drosophila. 



Tanaka refers to his case as differing from previously reported 

 cases of linkage in animals in that the sex differentiator is not 

 one of the genes involved, and in that the linkage is sometimes 

 only partial. However, he refers several times to a paper by 

 Morgan 6 in which it is clearly shown that three of the sex-linked 

 genes in Drosophila also show partial linkage to each other, inde- 

 pendently of their sex-linkup'. Punnett, 7 in referring to the 

 same paper, has said, "Morgan's experiments with Drosophila 

 suggest coupling of some kind between factors for eye color and 

 shape of wing, though both of these factors may show sex-limited 

 inheritance in other families." A study of the data referred to, 

 or of any of the similar data on Drosophila since published, will 

 show that these genes always show sex-linkage, and that at the 

 same time they always show linkage to each other when both can 

 be followed in the analysis. The two phenomena are not mutu- 

 ally exclusive, but both are always present. 



Both Tanaka (in a footnote) and Punnett refer to the latter 's 

 case in rabbits as the first example of linkage in animals not 

 involving sex. If the linkage between sex-linked genes is, for 

 some strange reason, not considered to belong in this category, 

 there are still at least two cases which antedate Punnett \s slightly. 

 A few months before Punnett 's paper appeared I had suggested 8 

 the possibility of linkage in mice. It now seems rather probable 

 that the relation in both mice and rabbits may really be that of 



