230 Linkage in Grammarus chevreiixi 



There appears from Allen and Sexton's work to be very little diifer- 

 ence in viability between the different classes. The recessives are perhaps 

 a trifle less hardy, but the differences from expectation thus produced in 

 the ratios are negligible. As we shall see, this factor can probably be 

 disregarded in the experiments here recorded, since the double recessive 

 classes, which should be the least viable of all classes, are not below 

 expectation. 



There is no possibility of confusing any classes save blacks and reds 

 {BOW and hCW). Occasionally the heterozygous blacks show a distinctly 

 russet or dark crimson tint. This, however, is in almost all cases wholly 

 distinct from the light scarlet of the reds. Very rarely specimens are 

 met with among the coloured-with-white class which are apparently in- 

 termediate ; whenever this is so the fact has been noted. The doubtful 

 cases almost all cropped up at the outset, and disappeared with practice. 

 In the coloured no-white classes {B, G, lu and b, G, w), however, I have 

 never met with heterozygotes which could not be at once classed with- 

 out hesitation; the colour stands out much more clearly when not against 

 the chalky background of the white pigment. If there is linkage between 

 B and G (and only between B and G) the ratio of black-with-white 

 (B, G, W) to red-with-white {b. G, W) should be the same as that of black 

 no-white (B, G, w) to red no-white {b, G, lu). Since the former ratio is 

 actually found to be very nearly equal to the latter, and since in this 

 latter ratio there is no possible source of error from confusion of classes, 

 we may assume that any error thus produced in the former ratio is 

 negligible, an assumption further justified by the rarity of doubtful 

 cases. 



Some broods were kept at room -temperature (10''-15°C.), others in 

 an incubator at 25°-26° C. Otherwise the treatment was the same for 

 all. The figures indicate that linkage (as in Drosophila — see Plough, '17) 

 is less intense at the higher temperature. There are further some slightly 

 aberrant ratios in the no-whit& (ww) classes. With these subjects I do 

 not propose to deal until I have further data ; here I shall only discuss 

 the question of linkage between B and G. 



If there is linkage between B and G of such strength that p repre- 

 sents the linkage and 1 -p the crossover- value in the female, while q 

 and 1 — 5 are the corresponding values in the male, we should expect 

 the following ratios (see Haldane, '19): 



Black -I- Black No-white {B, G, W -{- B, G, tv] 2 + i^g 



: Red -I- Red No-white [b, G, W+b, G, w] 1 - P*/ 



: Albino -f Colourless [B (b), c, W + B (b), c, w]...l. 



