2g2 Sturtevant. 



such that any member of a group shows linkage with any other member 

 of that group, but not with an>' member of either of the other two. It 

 may be argued that this is a special case, and that no such relations 

 exist in other groups. There are several answers to this objection. In 

 the first place, Drosophila has a smaller number of chromosomes (4 or 

 5, haploid number) than most other forms which have been extensivelj' 

 studied genetically, so that there is a better chance of finding linkage 

 in it (see Table XII). Secondly, cases of weak linkage may easily be 

 overlooked if one is not on the watch for them. Tliis is not the case 

 in Drosophila, because linkage is here always complete in the male, but 

 in forms where this relation does not exist, such as some plants (see 

 Gregoet 'lib, Bateson and Potxett '11c, Baub '12a, Pux^'ETT '13) 

 two genes may be in the same chromosome and stiU show no apparent 

 linkage. We have, in fact, genes in the same chromosome in Droso- 

 phila which appear to be practically independent in the female (e. g., 

 Y and W with E and Br in chromosome I, B with Ba and Sp in chromo- 

 some 11). In such cases the true state of affairs would appear only 

 when such genes were tested with genes lying between them. Thirdly, 

 there seems to have been no systematic search for linkage in other 

 forms. Most published data give the ratios for each character sepa- 

 rately, so that we get information available for linkage only when several 

 genes affect the same character. The writer has recently gone over 

 the literature on fowls and on peas from the point of view of linkage, 

 and has been surprised at how few data can be found there which bear 

 on this matter. I am sure that anyone who undertakes a similar in- 

 vestigation will agree that there may be much undiscovered linkage, 

 even in those forms which are supposedly very well worked out. 



However, linkage has actually been found in a rather large number 

 of forms. In the list which follows I have excluded all cases about 

 which I am in doubt, but most of these will be noted biiefly later. There 

 are included several cases of sex-linkage, which is merely a special case 

 of the more general phenomenon. 



Several cases reported as linkage have been excluded from this 

 list because it is more or less probable that they are really illustrations 

 of multiple allelomorphism (see discussion of this matter by Stubtevant 

 '1.3 b). Among these are certain colors in maize (Emehsox 'II, &c.), 

 which I formerly ('13 b) was inclined to suppose represented a true case 

 of linkage, but am not now certain of because of the facts reported by 

 Emerson ('13b); rabbits (Punkett '12 a), and mice (Sturtevant '12 b, 



