176 Outline of Genetics 



and more than two effective factors on each chromosome. At the 

 same time it is rather encouraging to believe that such a possibility 

 exists, so that eventually we may be able to obtain a race that is 

 homozygous for all or practically all of the desirable factors. 



2. A simple mathematical demonstration will show 

 that the distribution of F, individuals is quite what it 

 should be, represented by a symmetrical curve similar to 

 the curve of probabilities. 



It can be demonstrated rather rapidly that Jones's scheme 

 will satisfy the requirements on distribution of F2 individuals 

 with respect to hybrid vigor, and on the progressive loss of vigor 

 in the later inbred generations. It was seen that the heterozy- 

 gosis theory could account for these facts by the use of the simple 

 example, AAbbXaaBB. As a matter of fact, fig. 27 becomes the 

 exact mathematical equivalent of this example if we substitute 

 the ^c chromosome of the dominance scheme for A of the heterozy- 

 gosis scheme; the bD chromosome for b; the aC chromosome for 

 a; and the Bd chromosome for b. In this way it will be discovered 

 that the two schemes run exactly parallel in accounting for the 

 facts in every generation. 



In fact, this new theory, ''the dominance of linked 

 factors," seems altogether sound and natural. We 

 should rather expect that each chromosome would bear 

 several dominant factors conducive to vigor and several 

 recessives as well. 



Recently Collins (2) has presented some interesting consid- 

 erations bearing on this scheme of Jones's. Collins maintains 

 that in explaining hybrid vigor one should place the emphasis on 

 the suppression of deleterious recessive characters rather than on 

 the accumulation of dominant growth factors. This is merely a 

 change in the point of view. Collins further maintains, however, 

 that the dominance scheme can really account for the facts with- 

 out the assumption of linkage, provided a sufiicient number of 

 effective factors be assumed. There is some rather good evidence 

 to support these contentions of Collins. For the present, how- 



