Mutation and the Life Cycle 233 



type from which they arose. To sum up, then, most mutants are 

 recessive to the original stock. 



Mutation and the Life Cycle 



A gene mutation may occur at any stage of the life cycle of a 

 plant or an animal, and therefore in sporophytic or gametophytic 

 and in somatic or germinal tissue. Unless and until the mutation 

 is followed by a further mutation or by a reverse mutation of 

 the same gene, all the cells derived from the cell in which the 

 mutation occurred will contain the mutant gene. 



If a gene mutation occurred in only one gamete, that gamete 

 and the individual which resulted from it after fertilization 

 would have the mutant gene. If the new gene affected somatic 

 tissue and was a dominant mutation, the new character would 

 be noted immediately in the individual produced by the mutated 

 gamete. If the mutation was recessive, it would be hidden unless 

 it was in the sex chromosome of the heterogametic sex. 



If the nonlethal mutation occurred in one member of a pair of 

 homologous chromosomes before meiosis or before the "chromo- 

 some split" took place, half the gametes or half the megaspores 

 or microspores from the meiocytes in which the mutation oc- 

 curred would bear the mutated gene. In plants it would be 

 carried through the gametophyte and gametes to the new sporo- 

 phyte. If it was a gene that exerted an effect on the gameto- 

 phyte, it would be detected immediately whether it was a domi- 

 nant or a recessive mutation since the gametophyte is haploid. 

 If the character produced by the mutant was a character that 

 was visible in the sporophyte only, it would not show up in the 

 gametophyte and would appear in the sporophyte only if it was 

 dominant. If the mutation was recessive, it would be hidden in 

 the sporophyte, but would appear in the next sporophyte genera- 

 tion in about one-quarter of the offspring of that plant, pro- 

 vided the plant was self-fertilized. It is frequently difficult to 

 say just when or in what particular individual a mutation oc- 

 curred if it was recessive and affected only the diploid stage of 

 the life cycle, and if the plant or animal was one that was nor- 

 mally cross-fertilized. Such mutations might remain hidden for 

 several generations. 



Mutations may occur in somatic tissue at any stage and in any 

 part of the developing organism. If a mutation occurred in 



