vi MUTATION 181 



place in the fertilized (diploid) egg, the individual will again be in the 

 heterozygous condition unless both members of the homologous pair 

 concerned have undergone the same mutation. 



A number of cases of mutants appearing in the first instance in the 

 heterozygous form are on record. 



Nilsson-Ehle (1911) described such a case in oats. He found in 

 pedigree cultures of this cereal a small proportion (one in ten to twelve 

 thousand) of plants exhibiting certain atavistic features in respect to 

 the awns and the hairiness of the flower base. When these atavists 

 were self-fertilized they gave offspring of three classes, viz. normals, 

 atavists like themselves, and more pronounced atavists, in the 

 proportion of i : 2 : i. Obviously therefore the original semi-atavists, 

 as we may call them, were heterozygotes between the normal and 

 fully atavistic forms that is to say, they were produced by syngamy 

 of a mutated and a non-mutated gamete. 



Gates (1914) has shown that in the case of Oenothera rubricalyx, which 

 arose as a mutation of an 0. rubrinervis, the first rubricalyx individual 

 was a heterozygote between rubricalyx and rubrinervis, the new char- 

 acter rubricalyx being in this case dominant. Its heterozygous nature 

 was therefore not obvious from its external characters, but was only 

 disclosed by breeding. 



There is one great obstacle in the way of discovering the mode of 

 origin of mutations, and that is that a large number, probably the great 

 majority of them, are partially or completely recessive to the type condi- 

 tion, and therefore the heterozygotes are indistinguishable from the type 

 form. In these cases, since it is only the homozygous recessives which 

 exhibit the new character, this will make its first external appearance 

 in the homozygous condition. As moreover the new factor may have 

 been in existence a considerable time, and may have become widely 

 distributed, but invisible, owing to being always concealed in individuals 

 possessing also the dominant type character, these recessive mutants 

 are liable to appear suddenly in relatively large numbers, when sufficient 

 heterozygotes have accumulated in the population to allow of the meeting 

 between two mutated gametes to take place fairly frequently. 



Also it must be remembered that mutation, striking enough to be 

 recognized and investigated in the first animal or plant exhibiting it, 

 is rare. 



It must not be supposed, however, that mutation can only take place 

 in the germ-cells, nor is there any theoretical reason for supposing that 

 it should, like segregation, be connected with meiosis, except in that 

 limited class of mutations due to irregular distribution of the chromo- 

 somes or their constituents in the reduction division. 



An example of a mutation which almost certainly did not originate 



