NOTES AND LITERATURE 



LINKAGE IX THE SILKWORM MOTH 



One of the most striking recent developments in the study of 

 genetics is the discovery of linkage in many of those forms which 

 were supposedly thoroughly worked out. The most recent ex- 

 ample is a very interesting paper by Y. Tanaka. 1 entitled 

 "Gametic Coupling and Repulsion in Silkworms." The data 

 presented in this paper demonstrate the existence in the silk- 

 worm moth of a group of four pairs of linked genes. Following 

 Tanaka 's nomenclature we may designate these genes as follows: 

 N, which differentiates the larval color pattern known as 

 "normal" from that called "plain"; S, occurring in larvae 

 having the "striped" pattern, and epistatic to N; M, the differ- 

 entiator for the "moricaud" larval pattern, also epistatic to 

 N; Y, the gene which differentiates caterpillars with yellow 

 blood and yellow cocoons from the recessive whites. Of the six 

 possible combinations of these genes, taken two at a time, all 

 but NM and SM were made, and all showed linkage. F 2 

 "coupling" tests, i. e., from matings where both dominants 

 entered the cross from the same P x individual, were made for 

 SY and for MY. In each case there occurred cross-overs, or new 

 combinations of the characters, in such proportions as to lead 

 Tanaka to suppose the ratio of parental combinations to cross- 

 overs among the gametes to be about as 7:1. "Repulsion" 

 (where one dominant entered from each P 1 individual) F 2 

 results were obtained for N8 and for NY. In neither case did 

 any double recessives (cross-overs) appear, though over 3.000 

 caterpillars were obtained in the case of NY, and 224 in the case 

 of N8. From these data Tanaka concludes that the repulsion was 

 complete in these two cases. It has, however, been pointed out by 

 Morgan 2 that such results will be obtained if the linkage is com- 

 plete in one sex only. In Drosophila such "repulsion" crosses 

 never produce double recessives in F„, and it has been shown 

 that this is due to complete linkage in the male, crossing over 

 being frequent in the female between some pairs of genes. In 

 order to test this possibility it is necessary to mate doubly hetero- 

 zygous individuals to double recessives, when the gametic ratio 

 is obtained directly and without the complications present in 

 most F 2 results. It so happens that Tanaka reports two such 

 crosses, one for each sex, though he does not recognize their im- 



» Jour. Coll. Agr., Tohoku Imper. Univ.. Sapporo, Japan. V, 1913. 

 « Science, N. S., XXXVI, 1912. 



