204 READINGS IN BIOLOGICAL SCIENCE 



made, there was no necessity of searching for other types of analysis. With 

 the growing interest in the study of human inheritance, however, it was 

 increasingly realized that the classic methods could not serve in this 

 field. It became imperative to devise technics which would obviate the 

 necessity of knowing the precise genotypes of the parents, and which 

 would eliminate the need for the study of Y^ generations, back-crosses, etc. 



Once the need was felt, the technics were not long in appearing. In gen- 

 eral, such technics are based primarily on derivations of the frequencies of 

 the genes in the population, the derivations being made from the fre- 

 quencies of observable phenotypes. On the basis of such gene frequencies, 

 the results of various mass matings may be predicted. The many methods 

 now available have originated in scattered laboratories. Contributions to 

 this field have been made in England by Fisher, Haldane, Hogben, Pen- 

 rose and others; in Germany by Bernstein, Lenz, Wellish and others; and 

 in America by Burks, Wiener, Wright, Cotterman, Rife, Snyder and 

 others. In the course of the development of methods for analyzing hu- 

 man inheritance the number of generations required for the analysis has 

 been reduced first to two, and finally to but one, while the requisite knowl- 

 edge of the precise genotypes of parents has been gradually reduced and 

 finally eliminated entirely. 



It must not be thought that methods which lessen the required number 

 of generations available for study or which minimize precise genotypic 

 knowledge concerning parents are more desirable or more efficient than 

 the classic methods. It is merely that they must serve, as efficiently as pos- 

 sible, in a field in which test matings of precisely known genotypes are 

 not available. 



It will be readily seen that no single method can answer all the questions 

 about the genetic bases of human characteristics. Various technics are con- 

 cerned in solving the problems as to the number of pairs of factors in- 

 volved, whether these factors are acting as dominant, recessive, blending, 

 sex-linked, sex-influenced, lethal or multiple factors, whether or not epi- 

 static relationships are present, and whether the factors are linked or in- 

 dependent. In predicting the proportions of different types of offspring 

 to be expected from various mass matings involving specific phenotypes, 

 complications arise in that a single phenotype often includes several dif- 

 ferent genotypes. In linkage studies a heterozygous genotype may include 

 both coupling and repulsion phases. Hence it is necessary to provide suit- 

 able statistical corrections and allowances, since in human data such com- 

 plications can hardly be avoided. 



One of the points most frequently overlooked in the study of human 

 heredity is the matter of equilibrium in gene frequencies. It should now be 

 a commonplace that equilibrium in regard to the genotypes resulting from 

 a pair of autosomal factors exists when the homozygotes for one allele, 



