204 



CHAPTER 15 



the life cycle may be relativelj ill-adaptive at 



another whether or not these stages have 

 the same or different ploidies. It is. of 



course, the total adaptiveness of all these 

 separate features which determines the over- 

 all reproductive potential oi an individual. 

 Finally, it should he noted that in cross- 

 fertilizing populations, selection favors geno- 

 types which produce maximal fitness of the 

 population as a whole. Because selection 

 acts this way, it is possible that some por- 

 tion of the population receives genotypes 

 which are decidedly not advantageous. If 

 this is so. the same genetic components are 

 expected to be advantageous when present 

 in en her. more probable, combinations. 



* Selection Against Mutants 



Since the human being is primarily a diploid, 

 it is upon the diploid-produced phenotype 

 that selection principally operates. If one 

 asks, "What is the fate of mutants in the 

 gene pool?*' the answer must include knowl- 

 edge of the frequency with which the mu- 

 tants arise as well as their effects upon re- 

 productive potential in a diploid genotype. 

 Remember that the phenotypic effect of a 

 mutant gene depends not only upon the na- 

 ture of its allele but also upon its relationship 

 to the rest of the genotype. 



Let us consider, in turn, the fate in the 

 human gene pool of mutants whose overall 

 phenotypic effect is: dominant lethal; domi- 

 nant detrimental; recessive lethal; or reces- 

 sive detrimental, as influenced by selection 

 and mutation. 



Dominant lethal mutants are lethal when 

 heterozygous and are eliminated from the 

 gene pool the same generation they arise. 

 Accordingly, the biological fitness of such 

 mutants is zero. If the normal homozygote 

 (A i A ] ) is considered to have a selective 

 disadvantage of zero, then the dominant 

 lethal is at a complete selective disadvantage, 

 and the selection coefficient, s, is one. We 

 can readily see that the imitation frequency, 



ii, of this dominant lethal condition must 

 equal one half the frequency of affected in- 

 dividuals (A1A2), since each affected indi- 

 vidual has one mutant and one normal gene. 

 In the absence of special medical treatment. 

 retinoblastoma, a type of cancer of the eye, 

 is an example of such a dominant lethal in 

 man. 



A chondroplastic ( or chondrodystrophy ) 

 dwarfism is characterized by disproportion 

 — normal head and trunk size but shortened 

 arms and legs. This rare, fully penetrant 

 (see p. 73) disease is attributed to the pres- 

 ence of a gene in heterozygous condition 

 which therefore acts as a dominant detri- 

 mental mutant. Such dwarfs (A^A 2 ) are 

 known to produce only 20 per cent as many 

 children as normal people. Because of this 

 lower reproductive potential the A\A-> geno- 

 type selection coefficient is .8. 



In one study the frequency of A x A- 2 in 

 the population was found to be 10 dwarf 

 babies in 94,075 births. The dwarf children 

 in this sample must have resulted from nor- 

 mal parents who carried new mutations to 

 A ■> or from one normal and one dwarf par- 

 ent. The gene frequency, p, of A-< in the 

 population, therefore, must be 10 per 

 2(94,075) or .000053. From the incidence 

 of dwarfs known to have normal parents the 

 mutation frequency, u, to A 2 is .000042. If 

 the value s = .8 is correct, then p = u/s, or 

 .000042 .8, or .0000525, which is in excel- 

 lent agreement with the gene frequency (p) 

 value observed. Gene frequency for a 

 dominant lethal equals mutation frequency 

 (p = u) because s = 1; however, in the 

 present case s is less than one, so the gene 

 frequency is greater than the mutation fre- 

 quency. Actually the gene frequency for 

 dwarfism is not very much larger than the 

 mutation frequency, demonstrating the effi- 

 ciency of natural selection in eliminating 

 such mutants from the gene pool. 



The gene for juvenile amaurotic idiocy 

 {a-,) has no apparent effect when hetero- 



