152 Gametic Coupling [ch. 



and also as regards the factor B. Plants that are homozy- 

 gous In either of these allelomorphs have the normal distri- 

 bution of characters among their gametes, and they may 

 be heterozygous in C, Ry or in any of the other factors 

 recognized in the Sweet Pea without any departure from 

 the ordinary ratios being produced. 



The gametic series has been spoken of as 7 : i : i : 7 

 and these are the numbers which fit the observed result most 

 closely, but attention should at once be called to the possi- 

 bility that the series may in reality be 8 : i : i : 8. The 

 observed numbers are too small to enable us as yet to dis- 

 criminate between these two possibilities, though, as will be 

 seen when the nature of coupling is discussed, the signifi- 

 cance of the two series must be entirely different. It is 

 however to be noticed that the series of gametes necessary 

 to complete the whole system is thus either 16, or 16 + 2. 



In the next two examples of such partial coupling the 

 association is in groups of 32, or 324-2. Both these also 

 occur in the Sweet Pea. The first concerns the peculiar 

 sterility or contabescence of the anthers which has already 

 been mentioned as a recessive character. The second factor 

 is again a colour-factor. Among the various factors which 

 control colour in the Sweet Pea is one which causes the 

 appearance of a reddish purple spot in the axils of the 

 leaves, referred to already as the dark-axil factor. When 

 this factor is present (and the flowers are coloured) the 

 axils are dark ; when it is absent the axils are simply green 

 as they always are in ee;>^ 2/^- flowered plants^. At an early 

 stage in the Sweet Pea investigation it was noticed that 

 when a family contained plants differing in respect of sterility 

 and fertility of anthers as well as in respect of dark and 

 light axils, the plants with sterility in the anthers (having 

 coloured flowers) were almost always light-axllled, and 

 conversely the dark-axilled plants were almost always fertile 

 in the anthers. In such families, among the white-flowered 



* Dark axils sometimes exist in plants which have the flowers so 

 nearly white as to pass for real whites. Probably in all such flowers a 

 trace of colour is developed, and certainly in them the seed-coat is always 

 black as it is in all the Sweet Peas with coloured flowers. 



Plants raised from wild Sicilian seed were all purple bicolour in flower- 

 colour, and nearly all had dark axils, but a few had light axils. 



