242 



CHAPTER 18 



a race cannot have an average phenotype; 

 accordingly, it is futile to try to picture a 

 typical (average) member of any race. 



Other blood traits, besides ABO blood 

 group, whose genetic basis is understood. 

 are also useful in characterizing races. In 

 fact, it is valid to utilize any phenotypic 

 difference due to a genetic difference. For 

 example, in delimiting races one can employ 

 certain genetic differences in color of hair. 

 eyes, and skin, and differences in stature and 

 head shape; one should avoid using pheno- 

 ls pic ditTerences whose genetic basis is un- 

 proved, for the environment itself can cause 

 phenotypic differences (Chapter I ). Re- 

 member also that the same phenotypic result 

 may be produced by different genotypes be- 

 cause of gene interaction in dominance and 

 epistasis (Chapter 4). 



Knowledge of the distribution of genes 

 for ABO blood types in different populations 

 provides important information to geneti- 

 cists, anthropologists, and other scientists. 

 To what can the different distributions be 

 attributed? Since people do not choose their 

 marriage partners on the basis of their ABO 

 blood type, and since there does not seem 

 to be any pleiotropic effect making persons 

 of one blood type sexually more attractive 

 than those of another, it is very likely that 

 mating is at random with respect to ABO 

 genotype. However, in other respects some 

 evidence indicates that different ABO geno- 

 types do not have the same biological fitness. 

 Differential mutation frequencies can also 

 explain part of the differences in gene dis- 

 tribution. During the past few thousand 

 years the greatest shift in ABO gene fre- 

 quencies of different populations has prob- 

 ably been the result of genetic drift and 

 migration. In fact, the paths of past migra- 

 tions can be traced by utilizing — along with 

 other information — the gradual changes in 

 the frequencies of ABO and other blood 

 group genes in neighboring populations. 



It has already been mentioned (p. 209) 

 that different paracentric inversions are 

 found in natural populations of D. pseudo- 

 obscura. All oi these flies are very similar 

 phenot\ pically, even though their chromo- 

 somal arrangements are different. Sample 

 populations of this fly in the southwestern 

 part of the United States (Figure 1S-1) 

 have been studied to determine the relative 

 frequency of these inversions." California 

 populations proved to be rich in the inver- 

 sion types called Standard and Arrowhead. 

 Eastward, in nearby Arizona and New Mex- 

 ico, the populations contain relatively few 

 Standard and Pikes Peak chromosomes, 

 most chromosomes having the Arrowhead 

 arrangement. Finally, in still more easterly 

 Texas, one finds almost no Standard and 

 some Arrowhead with most chromosomes 

 being of the Pikes Peak type. 



The shift in the frequency and type of 

 inversions in the three different geographic 

 regions cannot be explained as the result of 

 differential mutation, since the spontaneous 

 mutation rate for inversions is extremely 

 low. Moreover, since there is no indication 

 that the gene flow among these populations 

 has changed appreciably in the recent past, 

 migration rates have probably had a rela- 

 tively small influence upon genotypic fre- 

 quencies; there is also no indication that 

 genetic drift has had a major role in causing 

 the differences in inversion frequency in the 

 three areas. These observations lead us to 

 suppose that the primary basis for these pop- 

 ulation differences lies in the different adap- 

 tive values which different inversion types 

 confer on individuals in different territories. 

 Despite the absence of any obvious morpho- 

 logical effects, these inversions prove to have 

 different physiological effects in laboratory 

 tests; different inversion types survive best 

 in different experimental environments. Since 



1 Bused upon work of Th. Dobzhansky and col- 

 laborators. 



