MUTATIONS 269 



Morgan. Without doubt it is caused by some irregularity in the proc- 

 ess of fertilization. Homeosis, or the replacement of one organ by 

 another, is known to have followed mutilation. Examples of the modi- 

 fication of characters by environmental conditions are given in Chapter 

 II. There are many similar variations in animals, none of which are 

 hereditary. However, we shall again refer to the possibility of somatic 

 mutations in animals. 



There is no direct evidence as to the cytological time of factor muta- 

 tions, but the stage in the germ cell cycle of animals at which factor 

 mutations are most likely to occur would seem to be shortly before 

 or during the process of maturation. This is indicated by the sporadic 

 appearance of mutants. The first observed mutation in Drosophila 

 ampelophila was white eyes, which were found in a few males among 

 several hundred individuals in a pedigreed red-eyed race. Similarly 

 with other sex-linked mutant characters that have been observed in this 

 species, they have appeared either singly or at most in a few individuals. 

 Had these mutations occurred at an earlier stage in the germ cell cycle, 

 more gametes would have been affected and more mutant individuals 

 would have been found. Obviously the length of time that must elapse 

 before a factor mutation can manifest its existence depends upon two 

 things in addition to the stage in the germ cell cycle at which it occurred : 

 (1) its relation to its normal allelomorph, i.e., whether it is dominant or 

 recessive; (2), its relation to sex determination, i.e., whether it is sex- 

 linked or not. A mutation from W to w in an X-chromosome of a normal 

 male Drosophila would have produced a heterozygous red-eyed female in 

 the next generation and no white-eyed flies whatever. One-fourth of the 

 progeny of such a female would in turn be white-eyed if she mated with 

 a normal male. Similarly with any non-sex-linked recessive character 

 which upon its first appearance in pedigree culture is found in more than 

 a single individual the probable order of events is as follows. A muta- 

 tion occurred in a single germ cell of a single individual, which mated 

 with a normal individual, thus giving rise to one heterozygote among its 

 progeny. This heterozygous individual mated with a normal individual, 

 producing heterozygotes among one-half of their progeny. Finally some 

 of these heterozygotes mated together and one-fourth of their progeny 

 bore the recessive mutant character. 



It would seem, therefore, that factor mutations in animals occur in 

 the germ cells shortly before or during maturation and the time of appear- 

 ance of a mutant character depends upon the relation of the mutant 

 factor to its normal allelomorph and whether or not it is contained in 

 the sex chromosome. 



In plants factor mutations may occur in any meristematic tissue as 

 well as in the germ cells. Observations on the occurrence of mutant 



