GENETICS 577 



The inheritance of certain traits depends not only on a single pair 

 of genes which determines the presence or absence of the trait but also 

 on a number of multiple factors which determine the extent of the 

 trait. For example, the presence or absence of spots in the coat of most 

 mammals is determined by a single pair of genes; the gene for the pres- 

 ence of spots (s) is recessive to the gene for solid color (S). The size and 

 distribution of the spots, however, are determined by a series of multiple 

 factors, and can be varied by selective breeding. Crossing two different 

 strains produces an F^ generation intermediate between the two parental 

 types and with little variability, and an Fo generation which is widely 

 variable, with some individuals having as many spots as the one grand- 

 parent and other individuals with as few spots as the other grandparent. 

 The term modifying factors has been suggested for multiple factors 

 which affect the degree of expression of another gene. 



282. Multiple Alleles 



In all of the types of inheritance discussed so far, there have been 

 only two possible alleles, one dominant and one recessive gene, which 

 could be represented by capital and lower case letters respectively. In 

 addition to a dominant and a recessive gene, there may be one or more 

 additional kintls of gene found at that same location in the chromo- 

 some that affect the same trait in an alternate fashion. The term multiple 

 alleles is applied to the type of inheritance in which there are three or 

 more different kinds of gene, three or more alternate conditions at a 

 single locus in the chromosome, each of which produces a distinctive 

 phenotype. Among the members of the species, of course, the alleles are 

 inherited in such a way that each individual has any two, and no more 

 than two, of the possible types of alleles. The members of an allelic 

 series are indicated by the same letter, with suitable distinguishing super- 

 scripts. 



One series of multiple alleles which affects coat color in rabbits in- 

 cludes the dominant gene C for normal coat color, the recessive gene c 

 which produces albino coat color when homozygous, and two other 

 alleles, c'' and c'"''. The gene c'', when homozygous, produces the "Hima- 

 layan" pattern of white coat over the body but with a dark color on the 

 tips of the ears, nose, tail and legs. The gene c'^'', when homozygous, 

 produces the "Chinchilla" pattern of light gray fur all over the body. 

 These alleles may be arranged in the series C, c'''', c^ and c, in which 

 each gene is dominant to the succeeding genes but recessive to the pre- 

 ceding ones. In other series of multiple alleles the genes may be incom- 

 pletely dominant so that the heterozygote has a phenotype intermediate 

 between those of its two parents, or one which is some combination of 

 the two parental phenotypes. 



Multiple alleles govern the inheritance of the human blood groups 

 O, A, B and AB (p. 544). The three alleles of the series, a*^, a^ and a, 

 regulate the kind of agglutinogen in the red blood cells (Table 12). 

 Gene a-^ produces agglutinogen A, gene a^ produces agglutinogen B 

 and gene a produces no agglutinogens. Gene a is recessive to the other 



