62 



CHAPTER 5 



consistent with the assumptions made, that 

 self- 01 cross-sterility is regulated by a single 

 pair of genes which form a multiple allelic 

 series. Some species prose to have fifty or 

 more multiple alleles forming a series re- 

 sponsible for self-sterility, group sterility, or 

 group incompatibility. 



^Multigenic Traits 



Up to now. the traits chosen to study genes 

 occur in clear-cut. qualitatively different al- 

 ternatives like flower color in garden peas, 

 or albinism vs. pigmentation, and various 

 blood types in human beings. These are 

 called discontinuous or qualitative traits be- 

 cause in each case an individual belongs 

 clearly to one phenotypic class or another. 

 Although the interaction of many or all genes 

 may ultimately be involved in the appearance 

 of a given phenotype, the phenotypic alter- 

 natives previously considered have been ef- 

 fected primarily by only one or a few pairs 

 of genes. Moreover, in these cases the non- 

 genetic environment had much less or no 

 effect upon the phenotypic differences in- 

 volved. 



For practical and theoretical reasons one 

 may also be interested in the genetic basis 

 for certain continuous traits like height of 

 corn or intelligence in man, for which there 

 are so many grades that individuals are not 

 separable into discrete types or classes. 

 Such traits are also called quantitative traits 

 because the continuous range of phenotypes 

 observed requires that an individual be meas- 

 ured in some way in order to be classified. 

 Are quantitative traits also determined ge- 

 netically? Let us make the simplest assump- 

 tion that quantitative traits differ from qual- 

 itative ones only in degree, the former being 

 due to the combined effects of many gene 

 pairs. In the case of multigenic (polygenic) 

 traits, although many phenotypic classes 

 would be made possible by the action of 

 multiple gene pairs, the effect of any single 



pair would be difficult to distinguish. Con- 

 sequently, since each pair of genes would 

 contribute only slightly toward the expres- 

 sion of the quantitative trait, one would 

 expect the effect of environment to be rela- 

 tively larger than that of any single gene 

 or gene pair. The large effect of fertilizer 

 upon corn ear si/e and of diet upon height 

 in human beings illustrate the importance of 

 environment in multigenic traits. 



A given trait may be determined qualita- 

 tively in certain respects and quantitatively 

 in other respects. For example, in garden 

 peas one pair of genes may determine 

 whether the plant will be normal or dwarf, 

 the actual size of a normal plant being de- 

 termined by multigenic interaction with the 

 environment playing a significant role. Sim- 

 ilarly, a single pair of genes can determine 

 whether a human being has a serious mental 

 deficiency or normal mentality, though nearly 

 all individuals have a degree of mental ability 

 which varies in a continuous way due to 

 environment and polygenes. 



If quantitative traits are determined multi- 

 genically, it ought to be possible to derive 

 other characteristics of them which are con- 

 sistent with actual observations by consider- 

 ing the same trait, first as a qualitative trait 

 (i.e., determined by one or two or three 

 gene pairs), and then as a quantitative trait 

 (i.e., determined by many gene pairs). Let 

 the trait be color, and the alternatives in P, 

 be black and white. Assume first that there 

 is no dominance or epistasis (see p. 51); 

 then, whether one, two, three, or many gene 

 pairs are involved, the F, will be uniform 

 and phenotypically intermediate (medium 

 gray) between the two P,. Examine, in 

 Figure 5-5, results of matings between F] 

 (by cross- or self-fertilization) in each case. 

 As the number of gene pairs increases, the 

 number of classes of Fj offspring increases. 

 As the number of classes becomes large, one 

 would expect environmental action to cause 



