322 Douhleness in Stocks 



(a) General considerations in regard to singleness and doubleness 

 apart from plastid character. 



The requirements 1, 2, 3, stated above, which concern the general 

 occurrence of singles and doubles apart from complications connected 

 with plastid colour, would be met if we suppose 



(1) That singleness — the dominant character — results from the 

 presence of two factors (X and F), doubleness from the absence of 

 either or both. 



(2) That in the eversporting forms these two factors are carried 

 only by the ovules and exhibit partial gametic coupling'. The propor- 

 tion of singles and doubles obtained points to a coupling either on 

 a 7:1:1:7 or on a 15:1:1:15 basis. Breeding experiments on 

 a much larger scale than it has yet been possible to attempt would be 

 required to determine with certainty which term in the series represents 

 the truth. But, as will shortly appear (see p. 324), certain results in 

 which plastid character has also to be taken into account are more 

 easily explained on the supposition of a 15 : 1 : 1 : 15 series, and we 

 may therefore adopt this value provisionally for the purpose of a working 

 hypothesis. 



On this view we may represent the eversporting zygote (so far as 

 singleness and doubleness alone are concerned) thus 



Xw Yy 



and its gametes, where 2n are required to exhibit the whole series, in 

 general terms thus^ 



' Coupling of the kind here indicated was first described by Bateson and Punnett in 

 the case of the Sweet Pea, where it was found that purple flower colour was partially 

 coupled with long shape of pollen. (See Bep. Evol. Committee, III, 1906, p. 9, and 

 IV, 1908, p. 3.) Several other instances of this kind are now known. For reference to 

 some of the more recently investigated cases, see Bateson and Punnett, I'roc. Roij. Soc. 

 Series B, Vol. 84, 1911, p. 3. 



- It is realised that the fact that the pollen of eversporting races appears only 

 to carry doubleness merely proves the absence of X and Y in comhination not of X alone 

 or r alone. The absence of either factor alone is deduced from the fact that all the 

 singles of an eversporting race appear to behave similarly, which would not presumably 

 be the case if some of the pollen carried A' or 1' and some did not. Were the pollen 

 thus heterogeneous we should expect eversporting singles to be of two kinds, yielding 

 different proportions of singles and doubles. We have no knowledge as to the cause of 

 this inability on the part of the eversporting pollen to carry the factors A' and 1'. If we 

 suppose that the quality maleness {M) in this case repels the factors X and Y, the 

 assumption, though it accounts for the fact in question, carries us no further. It is 

 almost unnecessary perhaps to add that when it is stated that the pollen is unable to 

 carry a particular factor, the meaning which the statement is intended to convey is that 

 the pollen is not carrying that factor in a form in which its presence can be detected. 



