232 Linkage in Gammarus chevreuxi 



The figures for A (broods at room temperature only) are : 

 r = '6397 ; mean crossover- value = 36'03 + 0'89 °/^. 



Those for G (broods at 25° C. only) are : 



r = "5608 ; mean crossover- value = 43'92 + 1"28. 



It will be seen that the difference between the crossover-values of A 

 and C is 7*89%. This is more than 6 times the larger probable error, 

 and is therefore significant. 



Group B represents the total of the broods first produced after trans- 

 ference from room temperature to 25° C. The eggs were already fertilized 

 and in the pouch before transference, so that the high linkage value 



r = "7149 ; mean crossover- value = 28*51 + 1*49 °/.^, 

 must be regarded as an accident. 



The few animals obtained from the back-cross -,' — '- x ,'^ — 



0, C, 10 0, c, w 



corroborate these findings. They are as follows : 



B, C, W B, C, w h, C, W h, G. w B (b), c, W B{b), r, w Total 



(6 broods) 13 14 2 10 32 27 



27 12 49 88 



Expectation p : 1 — p '• 1 



These show the same deficiency of both classes of reds. (They also 

 show the excess of individuals containing ww.) 



If we neglect the fact that the c, W and c, w classes are not exactly 

 half the total, we find that^ = f^ = '692, with crossover-value SO'S ± 4-87^, 

 which, with the few broods available, is a sufficiently close approximation. 

 It should in this connection be mentioned that different pairs vary con- 

 siderably in the strength of linkage which they exhibit. It should be 

 remarked that since the crossover-values for the back-cross are of the 

 *same order as those for the heterozygotes mated inter se, linkage must 

 be of about the same intensity in the two sexes. In this Gammarus 

 differs from Insects, and resembles Mammals. 



The abnormal ratios in Allen and Sexton's paper may all be explained 

 on the assumption of linkage of an intensity close to that here found. 

 E.g. their No. 6, p. 344, 2nd mating (linkage is not involved in the first 

 mating of this heading). 



If the composition were -j^ — x j^ — , then we should expect the ratio 



s, black : 1 — s, red : 1 , albino, where s = either p or q. 



The figures were BG, 31 ; 60, 8 ; B{h) c, 34; which gives s = '7949, 

 with c.o.v. = 20-51°/ . 



