182 CYTOLOGY CHAP. 



during gametogenesis is afforded by the origin of the peculiarly malformed 

 " cretin " sweet pea, described by Bateson and Punnet (1911) and 

 Punnet (1919). The malformation concerned is recessive to the normal 

 condition, and therefore only manifests itself in homozygous individuals. 

 The " cretin " arose in a pedigree culture, and was the only one of its 

 kind among a large number of direct and collateral ascendants and 

 descendants (excluding, of course, its own offspring). Had the mutation 

 occurred during gametogenesis two possibilities are open : (i) it might 

 have occurred during the gametogenesis of the grandparent, so that the 

 immediate parent of the cretin was heterozygous, though normal in 

 external appearance. In this case, however, it should have produced 

 one cretin among every four of its offspring, whereas it actually produced 

 only the one cretin and 51 normals. (2) The mutation may have occurred 

 during the gametogenesis of the immediate parent. If this were the 

 case, more than one gamete must have been similarly affected, since 

 the cretin itself, being homozygous, must have been produced by syngamy 

 of two such affected gametes. As, however, the parent plant produced 

 only one cretin among a large number of normals (the latter again pro- 

 ducing only normal offspring) it is plain that the number of mutated 

 gametes produced must in any case have been very small, and the chances 

 against two of them having united to form the cretin very great. The 

 evidence seems to indicate therefore that the mutation occurred in the 

 zygote cell, and affected both members of the homologous pair of chromo- 

 somes concerned. 



A well-known class of mutations occurring in somatic cells constitutes 

 the phenomenon of bud-variation in plants. Innumerable examples of 

 this could be quoted, but very few of them have been thoroughly investi- 

 gated. As an example of one which has been more fully worked out, 

 we may take a case described by Correns (1910) in Mirabilis. It was 

 found that plants with variegated foliage occasionally gave ' rise to 

 branches with pure green leaves. Seeds from flowers on these green- 

 leaved branches yielded 25 per cent variegated plants and 75 per cent 

 green. On further breeding, twenty-five of these 75 per cent of green 

 plants gave only green plants, and the remaining fifty gave again 25 per 

 cent variegated and 75 per cent green offspring. Thus it is plain that 

 the green branches appearing as bud-variations on the variegated plants 

 were heterozygous between variegated and green, green being dominant, 

 and produced the usual proportion of offspring for this type of hetero- 

 zygote, namely, 3 dominant : I recessive. 



In terms of cytology, we must suppose that the factor for chlorophyll 

 distribution exists in two forms one to produce a uniform distribution of 

 chlorophyll, giving green plants (G), and the other giving a patchwork 

 distribution, resulting in variegated plants (g). In the variegated plants 



