Foundations of the Mutation Concept. 19 



families in 1906 resulted in 2083 plants, all normal. Seeds for the 

 F 4 generation were saved from 14 individuals of one of the F 9 

 families numbering 181 plants. The F 4 from these 14 plants gave 

 1118 plants, one of which was the cretin. This appeared in a 

 family of 52 plants raised from the F 3 plant 304 6 /1906. Thirteen 

 sister plants yielded no cretin in a progeny of over 1000, nor did 

 one appear among the 2083 plants of the F 3 generation. The F s 

 of 195 plants grown in 1912 from 9 F 4 plants, also failed to produce 

 one, so it remains an isolated case. 



Punnett points out that under these circumstances it is 

 unlikely that the cretin originated from two germ cells, each of 

 which had lost the normal factor, for in that case the parent plant 

 would have been heterozygous, giving 25% cretins, and they would 

 also have appeared in the F 6 or in collateral families. Hence he 

 concludes, in agreement with the argument above, that the cretin 

 plant must have arisen through " some radical alteration in the 

 zygote after union between two normal gametes had already taken 

 place," and that it was " due to a change in the individual at some 

 stage after fertilization whereby the factor for the normal flower 

 was either dropped out or altered during the somatic divisions." 

 Since the germ cells of this plant were uniform, it is reasonable to 

 suppose that the change took place in the fertilized egg itself. 



Morgan (1919), in discussing this case, suggests that the 

 mutation occurred in one chromosome far enough back in the 

 germ track of the parent individual to give rise to pollen and ovules 

 (say in one flower) each carrying the mutated genes. But in that 

 case other germ cells would have been carrying these genes, and 

 these would surely have appeared later either in the heterozygous 

 or homozygous condition. 



As regards Drosophila, the great number of mutants it has 

 produced apparently all belong to one type, and behave in the same 

 way. They are nearly all classed as Mendelian dominants or 

 recessives in inheritance, though in a few dominance is variable and 

 subject to environmental influence. Morgan (1919), states that 

 only 12 are dominant out of over 150. These dominants have each 

 appeared in a single individual. Mutants from recessive genes, on 

 the other hand, usually came to light in about 25% of the offspring 

 of a pair, showing that the pair were both heterozygous for the new 

 factor, which must have arisen at least one generation earlier. 

 Whether both members of a pair of chromosomes undergo the 

 change simultaneously is unknown, since if it occurred in the egg 



