50 Note on a Case of Linkage in Paratettix 



heterozygous, as in Bg% . bC6 (or B@ . 06) than where it is only doubly 

 heterozygous, as in be® . Bed (or A® . Bd). 



In Table II the offspring of doubly and triply heterozygous females 

 is tabulated. Here there were 726 cross-overs out of 1570. giving a 

 cross-over value of 40*2 °/^. Where A was one of the factors concerned, 

 or on the above hypothesis the females were only doubly heterozygous, 

 there were 96 cross-overs out of 181, giving a cross-over value of 

 53"G + 2*5 7o (^he true value is presumably under 50 °/„). When A 

 was not concerned, or the females were triply heterozygous, there were 

 630 cross-overs out of 1389, giving a cross-over value of 45'4 + 9 °/^. 

 Hence in the female also linkage appears to be stronger in triple than 

 in double heterozygotes. 



These results are confirmed by an examination of the families where 

 both parents were of the same doubly (or triply) heterozygous composi- 

 tion. For example the matings 175, 176, and 177 (all A® .Bdx A®. B6) 

 gave: 



116 BB® and BA%, 43 AA®, 43 BBOd and BAdO, 2 AAOd, 



a typical F^ repulsion series. 



The type of linkage found is similar to though far less intense than 

 that found by Nabours (6) in Apotetticc, where cross-over values up to 

 12 °/q were found in the female and much smaller values in the male. 

 It is thus intermediate between the type found in Drosophila and 

 Bombyx, where no crossing-over occurs in the digametic sex, and the 

 type found in plants and mammals where the linkage is approximately 

 equal in the two sexes. 



Since according to Robertson (7) and Harman (8) the haploid number 

 of chromosomes in Paratettix is 6 or 7, it is somewhat surprising that 

 the loci of all the 15 factors so far discovered should be in one chromo- 

 some, which must be the case on Morgan's theory of linkage. Even if 

 only I, S, and © were in different loci, the odds against finding three 

 loci running in the same chromosome would be of the order of 48 to 1, 

 though somewhat lessened by the unequal sizes of the chromosomes. 

 This difficulty would be diminished if we assumed that the individual 

 chromosomes were very rigid, but that moderate linkage occurred between 

 chromosomes formed from adjacent parts of the spireme. This latter 

 possibility is suggested by certain cytological observations. 



The above deductions were only rendered possible by the unusual 

 fullness of Nabours' published records, which enable the complete 

 pedigree of any individual to be traced with the greatest ease. 



