218 



CHAPTER 16 



multiply .13 by 32. The resultant value of 

 about 4 represents a 40095 chance that the 

 ordinar) individual carried in heterozygous 

 condition a genetic load of detrimental mu- 

 tants which would have been lethal if homo- 

 zygous. In other words, on the average, 

 each person carried four lethal equivalents 

 in heterozygous condition, or, lour times the 

 number of detrimentals required to kill an 

 individual if the genes involved somehow 

 became homozygous. 



The preceding analysis did not reveal the 

 number of genes involved in the production 

 of the tour lethal equivalents. These lethal 

 equivalents might have been due to the pres- 

 ence in heterozygous condition of four re- 

 cessive lethals, or eight mutants producing 

 50% viability, or sixteen mutants with 25% 

 viability, or any combination of detrimental 

 mutants whose total was four lethal equiv- 

 alents. Because of environmental improve- 

 ments (better housing, nutrition, and med- 

 ical care) since the last century, it is likely 

 that the effect of the same mutants in present- 

 day society would be expressed by some- 

 what less than four lethal equivalents. For 

 the same reason, the detrimental effects of 

 these mutants in heterozygous condition are 

 expected to be somewhat less at present than 

 they were a century ago. For example, in 

 the last century a particular hypothetical 

 homozygous combination having variable 

 penetrance and expressivity would have pro- 

 duced no detectable effect 25% of the time; 

 a detrimental effect — but not death before 

 maturity — 15% of the time; and death be- 

 fore maturity 60% of the time; today, the 

 respective values would be 50%; 10%; 

 40%. A century ago this combination 

 would have produced .6 of a lethal equiva- 

 lent; at present, the portion is .4. Notice 

 also that the detriment not lethal before 

 maturity would also have been reduced dur- 

 ing this period from 15% to 10% or, speak- 

 ing in terms of detrimental equivalents, what 

 had been .15 would now be .10. Appar- 



ently the genes responsible for lethal equiva- 

 lents and for detrimental equivalents must 

 be the same, at least in part. 



It is also apparent that present-day man 

 carries a genetic load. Some of those mu- 

 tants transmitted to him arose in his parents 

 (probably two of each five zygotes carry a 

 newly arisen mutant, as mentioned on p. 

 192), and others arose in his more remote 

 ancestry. It has been calculated :i that, on 

 the average, each of us is heterozygous for 

 what is probably a minimum of about eight 

 such mutant genes. This genetic load does 

 not include the mutants carried in homozy- 

 gous condition. What happens to this load 

 of mutants in successive generations? 



Balanced vs. Mutational Loads 

 To predict, in a general way, the fate of the 

 "usual" mutant in the population, it is neces- 

 sary to determine its "usual" phenotypic 

 effect. 4 Since the typical mutant is detri- 

 mental when homozygous — at least to some 

 degree — the homozygous condition tends to 

 eliminate it from the gene pool. But two 

 opposite effects are possible for mutants 

 when heterozyg3us (see Chapter 15): either 

 the heterozygote is superior to both homo- 

 zygotes (as is found for the sickling-causing 

 gene in malarial countries), or the hetero- 

 zygote is somewhat inferior to the nonmu- 

 tant homozygote (as is true for most point 

 recessive lethal heterozygotes). In the for- 

 mer case the heterotic effect tends to in- 

 crease the frequency of the mutant, and both 

 the normal and mutant genetic variants are 

 retained in the population gene pool at equi- 

 librium. A population which normally re- 

 tains more than one genetic (or chromo- 

 somal) alternative in its gene pool, there- 

 fore, exhibits balanced polymorphism in its 

 phenotypes. This component of the genetic 

 load is balanced, and is, therefore, a balanced 



■■ By H. J. Muller and by H. Slatis. 



•» See B. Wallace (1963), J. F. Crow and R. G. 



Temin (1964), and Th. Dobzhansky (1964). 



