1911.] On ike Inter-relations of Genetic Factors. 



7 



The heterozygote Ab . aB forms only two types of gametes, and the hetero- 

 zygote AB . ab gives the coupled series of four types. Since the same factors 

 are involved in both cases it looks possible that the difference in behaviour 

 may be a consequence of the difference in the geometrical positions of the 

 factors relative to the planes of some critical division or divisions in the two 

 cases. There may, in fact, be a difference of polarity between the two kinds 

 of heterozygote. 



The increase in number of the two types of cell, AB and ab, may be reached 

 by proliferation of the two primordial cells of those two types. It may further 

 be remarked that though the numbers characteristic of coupled systems 

 cannot be produced by simple dichotomies, they can readily be represented as 

 produced by a series of periclinal and anticlinal divisions. For example if 

 AB 1 by periclinal division give off AB 2 , and this by anticlinal division become 

 two cells, which again divide periclinally and anticlinally, seven cells AB are 

 formed ; by repetition of the same processes 15 are formed, and so on. 



Systems of three Factors. — From the list given above it will be seen that in 

 the sweet pea we know two distinct factors, viz., erect standard and long- 

 pollen, which may be severally coupled with a third factor, that for blue 

 colour. Here, therefore, we meet a system of inter-relationship between three 

 pairs, and special interest must attach to a determination of the genetic 

 properties of plants heterozygous for all three. (The distribution of the 

 factors for fertile anthers and dark axils, so far as evidence goes, is 

 independent of this system of three pairs, so that, for the present, fertility of 

 anthers and axil-colour can be left out of account in a consideration of the 

 triple system.) 



A plant heterozygous for B (blue), L (long pollen), and E (erect standard), 

 can be made by any of four possible combinations. 



(1) EBL x ebl. 



(2) EB1 x ebL. 



(3) Ebl xeBL. 



(4) eBl xEbL. 



All these various types of combinations are now either made or being 

 made, but as yet we are only able to give the result in the case of No. 3. 

 In it B and E repel, and B is coupled with L on the 7 : 1 system. The 

 coupling of B with L, since they come in together, may seem to be what 

 the general trend of the evidence leads us to expect, but the fact that 

 E is repelled by B rather than by L is worthy of special notice, for we 

 know that E and L repel each other when B is not present. It suggests 



