344 GENETICS I 



As to the actual time, place and manner of the occurrence 

 of gene mutations, the following may be said. At a given 

 time a mutation occurs in but one of the two genes of any 

 gene-pair contained in a cell. Furthermore, it has commonly 

 been held that a mutation occurs at a given time in but a 

 single gene of the long chain of genes (figure 36). As 

 shown on page 334, however, this idea requires certain 

 qualifications. It has recently been shown that mutations in 

 several different genes may accompany chromosome break- 

 age. And under radiation several genes of a particular cell 

 may show mutation, possibly due to chromosome breakage 

 of which there is no other evidence. Usually, however, only 

 mutations of single genes have come to the notice of in- 

 vestigators at a given time and in a single cell. 



Since most mutations are recessive in their effects, and 

 since only one gene of the pair of genes present is mutated, 

 as a rule the mutation produces no manifest effect on the 

 individual in which it occurs. The fact of its occurrence can- 

 not be detected in that individual. It is only when, by the 

 processes of mating in ordinary reproduction, two of the 

 recessive mutations get together in the same pair, in some of 

 the descendants of this individual, that the effect of the 

 mutation appears to view. Thus when the mutation be- 

 comes manifest, it has actually occurred, as a rule, at least 

 two generations earlier, in one of the ancestors of this in- 

 dividual. This relation introduces much difficulty into the 

 study of the time and place at which mutations occur. Many 

 mutations occur that do not come to light for several or 

 many generations; indeed there must exist in any organism 

 many mutations that have never come to light. In the course 

 of generations a large number of recessive mutations may 

 have collected in the chromosomes, none of them producing 

 any effect, because each is accompanied by a normal or un- 

 mutated gene of the same pair. If now inbreeding — the 

 mating of close relatives — occurs, a number of these mu- 



