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 ne 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, 7.e., whether it is dominant or 
recessive; (2), its relation to sex determination, 7.e., whether it 1s sex- 
linked or not. A mutation from W to win 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 
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