6 



Mr. W. Bateson and Prof. E. C. Punnett. [Mar. 2, 



proved to occur. We know also that repulsion occurs between long pollen 

 and the erect standard in families where blues are not present but hitherto 

 we have not had an opportunity of determining the system of coupling 

 followed by this pair of factors. 



Several curious and important lines of inquiry are thus opened up. As to 

 the actual meaning or nature of coupling or repulsion there is no clue. 

 The fact, however, that the mode in which factors are combined in the 

 original parents can influence the distribution of the factors among the 

 gametes of T\ introduces a new conception into genetic physiology. 

 Eeciprocal matings give identical results, so no mere question of maternal 

 influence is involved. 



In attempting to form any conception of what actually happens in coupling 

 or repulsion, and of the cause which determines that the one phenomenon or 

 the other shall occur, we are met at once by the difficulty that we do not 

 yet know how or when the system 1 AB : 1 aB : 1 Ab : 1 ab, which we regard 

 as the normal distribution for two pairs of allelomorphs, is produced. 

 There is as yet no proof that the segregation of both pairs of factors occurs 

 at one division, or that that division is one of those which we regard as 

 specially concerned in maturation. Now that we know of a series involving 

 as many as 256 terms (127 + 1 + 1 + 127) it is most difficult to conceive that 

 such a system can be produced in the maturation-divisions of the ovarian 

 tissue of such a plant as a sweet pea. We may well be tempted to look 

 much earlier in the developmental processes for the establishment of these 

 differentiations, and it is not impossible that they may be established as 

 early as the embryonic constitution of the sub-epidermal layer itself. As is 

 known, this layer is— in most higher plants, at least — the exclusive source of 

 the germ-cells, a fact which leads to those remarkable consequences which 

 Baur has discovered in the genetics of variegated plants. Bemote as this 

 possibility admittedly is, in a problem of such extreme difficulty even 

 improbable suggestions are worthy of consideration. 



If we knew how the normal distribution, 1 AB, 1 aB, 1 Ab, 1 ab, is brought 

 about we might surmise by what modification the other distributions are 

 created. As it is, we can only say that in repulsion the heterozygote AaBb 

 gives off germ-cells of two types, Ab and aB, whereas in a coupled system there 

 are four types, AB, Ab, aB, ab, the two terms AB and ab being represented 

 7 times, 15 times, etc. One step further may perhaps be gained by arranging 

 the symbols so as to represent the combinations more accurately to the eye — 



1. AbxaB. 2. AB x ab. 



I I 

 Ab . aB. AB . ab. 



