MUTATION 249 



appeared quite often, represent, it seems, a peculiar 

 change whose relation to the changes that stand behind 

 other mutant characters is not yet worked out. 



In plants the best evidence is that reported by Emerson 

 for Indian corn. Emerson has shown that when a race 

 of corn {Zea mais) having red cobs and red seeds is 

 crossed to a race having white cobs and white seeds only, 

 the two original combinations appear in the second (F2) 

 generation giving plants with red cobs and red seeds and 

 plants with white cobs and white seeds. Either a single 

 factor determines that both cob and seed are red in one 

 case and white in the other, or if the color of each part 

 is due to a separate factor these factors are completely 

 linked. Now striped seeds with white cobs sometimes 

 mutate to red seeds and red cobs. The new combination 

 (red and red) acts as a unit toward the other known com- 

 binations. Therefore a single factor must have changed, 

 for, if not, mutation must occur in two (or more) closely 

 linked factors, i.e., for seed and cob color at the same time, 

 which is highly improbable. 



In forms propagating by sexual methods it cannot 

 be told whether mutation has occurred in one locus or in 

 both homologous loci at the same time, because in the egg 

 one of each pair of genes is lost in the polar body, and 

 irrespective of whether one or two mutated genes were 

 present only one member of the pair is left in the ripe egg ; 

 and in the sperm the chance of any one sperm reaching 

 the egg is so small that it is unlikely that the difference 

 between one sperm or two sperms having the mutated 

 locus could be detected. It is true that of the twelve domi- 

 nant mutants that have appeared in Drosophila each 

 appeared at first in a single individual — never two — ^which 

 might appear to favor the single locus view, but this evi- 

 dence is too meagre to be significant. Mutants from reces- 

 sive genes usually come to light in about a quarter of the 

 offspring of a given pair. This means that both parents 

 were heterozygous for the mutant gene, but this gene 



