42 I The Process of Evolution 



to affect crossing-over. In evolutionary terms, it would appear that 

 these factors directly influencing crossing-over per se have not been 

 important in the control of recombination. When the amount of re- 

 combination is reduced in certain organisms, presumably by the 

 operation of natural selection, other mechanisms usually are in- 

 volved, such as inversion, translocation, or elimination of meiosis 

 altogether (in apomictic organisms; see Chap. 9). 



THE EXPRESSION OF GENES 



The action of genes in an individual organism has proved to be quite 

 variable. Genes in a population are rarely found in only two alterna- 

 tive states or alleles. Rather, a system of multiple alleles appears to 

 be the common system governing most characteristics, e.g., blood 

 groups in animals, incompatibility systems in plants, coat color in 

 mammals, flower color in plants, etc. Interaction among the genes 

 in the genotype or of the developmental pathways resulting in the 

 phenotype produces complex genetic ratios. For example, where 

 pigment systems leading to the formation of a particular color are 

 involved, several genes may control different steps in the elaboration 

 of the pigment. If any one is missing, color is lacking. Such genes 

 are called complementary genes. 



On the other hand, there are many situations where the expression 

 of the gene at one locus masks the expression of another gene. This 

 is known as epistasis; the epistatic gene masks or prevents the ex- 

 pression of a hypostatic gene. In chickens, for example, the Leg- 

 horn white color is epistatic to many genes affecting color and 

 pattern. If it is present, no color but white will be expressed. It is 

 obvious that complementary effects and epistasis, as well as other 

 sorts of modified expression, are related developmentally. A similar 

 phenomenon occurs where there appears to be one gene or a group 

 of genes, each with relatively small effect, that operate to alter the 

 action of a gene with major effect. These "minor" genes are known 

 as modifying factors. 



It often becomes necessary to specify the type of expression of 

 genes because, for many factors, this is variable. For certain factors, 

 all or almost all individuals with the same genotype develop a 

 characteristic phenotype that distinguishes them from individuals 

 with other genotypes in a certain range of environments. The genes 

 in such cases are said to have high penetrance, since most indi- 

 viduals carrying the gene possess the trait. Other genes do not 

 always produce a detectable phenotypic effect in the individuals 

 that carry them in a given environment. These are genes of low 



