HUMAN HEREDITY AS REVEALED BY PEDIGREES 447 



gate in Mendelian ratios in subsequent generations, such character 

 differences are determined by genes. (2) Allelomorphic differences 

 have arisen through the process of gene mutation, a process so well es- 

 tablished for lower organisms. These postulates are justified, for it is 

 hardly likely that so universal a method of heredity as the Mendelian 

 method operates for the rest of the organic world and not for man. 

 Also, the method of gene mutation is the only method by means of 

 which allelomorphic differences are yet known to arise, and it is hardly 

 likely that man is an exception to the rule. 



The rate of mutation for man has been worked out for various races 

 of mankind. The number is almost the same in the various races. 

 For every 115 normal genes in the Russian race there are about 5 more 

 mutant genes than in the Negro race. "If 6,000 generations," says 

 Danforth, "represents anywhere nearly the time since these two 

 groups, along with others, diverged from a common ancestor, these 

 additional mutant genes must have been acquired in this length of 

 time These figures indicate that in the course of 6,000 genera- 

 tions at least 5 in every 115 gene lines which have persisted underwent 

 mutation in this one direction. Since there is no evidence of selection, 

 this may be presumed to be the rate of mutation in this particular 

 germplasm, from which it follows that there has been, on the average, 

 one mutation in every generation for each 138,000 genes. If, as some 

 might be inclined to think, the two germplasms have passed through 

 more than 6,000 generations since they separated, the rate of mutation 

 has been less than this." 



Thus we see that mutations, though infrequent, continue to appear 

 at a slow and steady rate. Like the mutations in Drosophila and 

 those of other animals studied, the majority of mutations in man are 

 poor, many are indifferent, and very few are good. It has evidently 

 taken a long time for man to acquire the better hereditary traits he 

 now possesses. In the past, natural selection has tended to eliminate 

 most of the undesirable mutants, especially the dominant ones; but 

 under civilized conditions mankind has done much to nullify the good 

 work of natural selection by doing everything in his power to preserve 

 the poorer mutants and help them to multiply. 



SOME HEREDITARY TRAITS IN MAN 



Studies of nearly 200 human traits have been made. In the ma- 

 jority of cases they can be classified as dominants, ordinary recessives, 

 and sex-linked recessives. The latter are the easiest to detect, for 



