30 TJte Origin of a Mutation in the Sweet Pea 



the cretin was discovered the numbers saved were fewer than could 

 have been wished. For various reasons the sowing of these seeds was 

 postponed until 1912. They germinated poorly and the F^ families from 

 9 plants consisted of but 195 individuals (cf Table I). No cretin how- 

 ever was found among them. Seed was collected from the four plants 

 Nos. 145-148 and sown in 1913. The 135 plants which resulted were 

 all normal. The series of experiments was not continued beyond the F^ 

 generation. 



The cretin then had its origin in a single seed of the F^ plant 

 No. 30471 906 and was the only case of its kind in a family of 52 plants. 

 None of the 13 sister plants of 304^ produced a cretin among a progeny 

 of over 1000, nor did such a plant appear in the large F2, generation 

 of 2083 individuals of which 304*^ was a member. Though the F^ gene- 

 ration raised from the sister plants of the cretin was not large, yet four 

 of the families were certainly of sufficient size to have produced cretins 

 had they been heterozygous for this simple recessive character. The 

 evidence taken together renders it unlikely that the origin of the cretin 

 was due to the meeting of two germ cells which had each lost the 

 normal factor. Were the mutation of germinal origin we should be 

 inclined to place its occurrence in the parent plant of 304**, viz. in the 

 F^ plant 309^ and we should have expected cretins to form about 25 ^\^ 

 of the family in which they first appeared. Again we should have 

 looked for their further appearance in some of the F^ families grown 

 from sister plants of the cretin itself We are led therefore to suppose 

 that the appearance of this peculiar form is due to a change in the in- 

 dividual at some stage after fertilisation whereby the factor for the 

 normal flower was either dropped out or altered during the somatic 

 divisions. 



It has been assumed that the cretin always behaves as a simple 

 recessive, and some evidence has already been published in support of 

 this assumption. More extended experience during the past few years 

 has served to confirm this view. Crosses between cretins and normals 

 of various families have been carried to the F^ and F^ generations and 

 in no case is there any reason for supposing that the cretin behaves 

 otherwise than as a simple recessive. A brief summary of the results 

 is given in Table II. Over a period of seven years 80 families have been 

 bred in which cretins occurred. Out of 5520 plants recorded in these 

 families 4198 were normal and 1322 were cretins — a proportion not far 

 removed from the expected ratio 3:1. There is therefore no reason for 

 supposing that the difference between the cretin and the normal is other 



