242 



CHAPTER 28 



cestry. It has been calculated^ that, on the 

 average, each of us is heterozygous for what 

 is probably a minimum of about eight mu- 

 tant genes received in these ways. What 

 happens to this load of mutants in successive 

 generations? 



In order to predict, in a general way, the 

 fate of the "usual" mutant in the popula- 

 tion, it is necessary for us to determine the 

 phenotypic effect of the "usual" mutant. 

 Since the typical mutant is detrimental, at 

 least to some degree, when homozygous, the 

 homozygous condition tends to eliminate 

 the mutant from the gene pool. But we have 

 seen in Chapter 27 that there are two oppo- 

 site effects possible for mutants when hetero- 

 zygous — either the heterozygote is superior 

 to both homozygot.'s (as is found for the 

 gene for sickling in malarial countries), or 

 the heterozygote is somewhat inferior to the 

 nonmutant homozygote (as is true for most 

 heterozygotes for point recessive lethals). 

 In the former case, the heterotic effect would 

 tend to increase the frequency of the mutant, 

 so that both the normal and mutant variants 

 would be retained in the population at equi- 

 librium. Such a population, which normally 

 retained in its gene pool more than one genie 

 (or chromosomal) alternative, would, there- 

 fore, exhibit balanced polymorphism. In the 

 case where the heterozygote shows detriment, 

 the heterozygous condition would increase 

 the rate at which the mutant was eliminated 

 from the gene pool. 



Experimental evidence in Drosophila '* and 

 a statistical analysis of data for man ^ sup- 

 port the view that the great majority of point 

 mutants are detrimental when heterozygous. 

 We shall, therefore, consider that the usual 

 mutant is not heterotic when heterozygous, 

 but is detrimental to a degree that is some- 



3 By H. J. MuUer and by H. Slatis. 

 "• Based upon work of H. J. Muller and coworkers, 

 C. Stern and coworkers, J. F. Crow and coworkers, 

 I. H. Herskowitz and R. C. Baumiller, and others. 

 ^ Based upon an analysis by N. E. Morton. 



what less than it would be when homozygous. 

 How is such a mutant gene eliminated from 

 the population? It need not be ehminated 

 by producing the death of an individual, 

 although it is sometimes removed this way. 

 A more general way to express the removal 

 of a mutant gene from the gene pool is by 

 genetic death, the failure to produce de- 

 scendants carrying the mutant. Thus, all 

 the genes in an individual, whether they be 

 normal or mutant, suffer genetic death if 

 that individual fails to produce children. 

 Since mutants are stable, they are removed 

 from the gene pool only by genetic death, or, 

 rarely, by mutation of the mutant. A per- 

 son carrying a dominant lethal like retino- 

 blastoma suffers genetic death (as well as 

 physical death). In this case, the mutant 

 gene is eliminated from the population the 

 generation in which it arises by mutation; 

 it shows, therefore, only one generation of 

 persistence. The dominant mutant for achon- 

 droplasia produces an average reproductive 

 potential of .2 as compared to normal and 

 will persist for five generations, on the 

 average, before suffering genetic death. This 

 means that each generation, in a population 

 maintaining approximately the same size in 

 successive generations, the achondroplastic 

 individual has an 80% chance of not trans- 

 mitting the mutant. So, when this mutant 

 arises, sometimes it will fail to be trans- 

 mitted the very first generation, at other 

 times it will suffer genetic death the fifth 

 generation, and at still other times at the 

 tenth. But, on the average, the mutant will 

 persist five generations before being lost. 

 You realize that even though genetic drift 

 and migration can cause fluctuations in the 

 frequency of the mutant, the principle of 

 persistence will still obtain. 



Consider the fate in the population of a 

 rare recessive lethal gene like that involved in 

 producing juvenile amaurotic idiocy. Each 

 time that homozygosis for this gene occurs, 

 it results in genetic death, and two mutant 



