44 I The Process of Evolution 



penetrance. It is possible also that the phenotypic expression of a 

 gene may be variable even though it is completely penetrant. If it 

 is relatively uniform in an essentially "normal" environment, the 

 gene has constant expressivity, but if there is interindividual varia- 

 tion in the trait, expressivity is variable. 



Studies of the manifold or pleiotropic action of genes, as well as 

 of systems of genes controlling the expression of particular character- 

 istics, have suggested that probably no character of an organism is 

 controlled by only one gene, and, conversely, every gene in the 

 genotype of an organism affects a great many ( if not all ) characters 

 in the complex process of producing the phenotype. But it seems 

 likely that the amino acid sequence of a specific polypeptide is de- 

 termined by one and only one gene. 



MUTATION 



It is often possible in controlled crosses to identify specific genes 

 affecting particular characters that show discontinuous variation. 

 The most obvious characteristic of such major genes is that they 

 change or mutate— indeed, that is the only way in which their exist- 

 ence may be detected. Mutations occur spontaneously at varying 

 rates; they also may be induced by treatment of the organisms with 

 ionizing radiation, ultraviolet light, or various chemicals. These 

 treatments appear to affect the DNA more or less specifically. The 

 discussion of spontaneous mutation rate is difficult because there are 

 several ways in which the rate may be expressed. As far as is known, 

 the mutational event is random; it is not possible to specify what 

 gene will be affected or to assign the cause of a given mutation to a 

 specific mutagenic agent. It would be desirable to know the chance 

 of occurrence of a mutation per cell per division, which expresses 

 change with respect to time. This is very difficult to determine in 

 other than microorganisms. Even with bacteria, what is measured is 

 phenotypic change which may involve more than one gene or mu- 

 table unit. Rates of from 10"" to 10 ~^ have been measured in micro- 

 organisms. 



In multicellular organisms, rates must generally be measured dif- 

 ferently (except where tissue culture is possible) since the criterion 

 available is the number of gametes producing mutant individuals 

 per generation of the organism. Thus individuals, not gametes, are 

 counted. In the gonads a mutation occurring in a gamete-producing 

 cell may have many or few mutant daughters, depending upon when 

 in gametogenesis the mutation took place. The mutation rate per 

 generation varies with the gene studied but averages about 10~'^. 



