234 



CHAPTER 27 



Consider the three genotypic alternatives, 

 AA, AA', A' A' relative to their phenotypic 

 effects. If A is completely dominant to A' , 

 and A' A' is less vigorous than AA and AA' , 

 homozygosis resultant from breeding of AA' 

 will clearly lead to detriment. This remains 

 true even if A is incompletely dominant to 

 A' or shows no dominance to it. In all these 

 cases, the heterozygote is superior to one of 

 the homozygotes. The second possibility 

 remains, however, that the heterozygote, AA' , 

 may be of greater adaptive value than either 

 homozygote. Suppose, to illustrate this, 

 that A is pleiotropic and has a relatively very 

 adaptive effect with respect to trait M, but a 

 relatively less adaptive effect with respect to 

 trait N, while its mutant allele has the re- 

 verse effect, namely, being relatively less 

 adaptive for M, and relatively more adaptive 

 for N. The heterozygote would, in the event 

 of no dominance, be superior to either homo- 

 zygote. Heterosis can be produced, there- 

 fore, if the heterozygote is superior to either 

 one homozygote or both homozygotes. 



The first type of heterotic effect can be 

 demonstrated by crossing two pure lines 

 homozygous for different detrimental reces- 

 sives {A A bb CC dd EE X aa bb CC DD EE). 

 The Fi {Aa bb CC Dd EE) will be uniform 

 yet more vigorous (having four different 

 normal alleles) than either parent (each of 

 which had only three different normal 

 alleles) because the dominant alleles hide the 

 detrimental effects of the recessive alleles. 



The second type of heterotic effect can be 

 illustrated in human beings. As mentioned 

 in Chapter 10, homozygotes for the gene 

 for sickle cell anemia {A' A') usually die from 

 anemia before adolescence. AA individuals 

 are of normal blood type, while AA' indi- 

 viduals are either normal or have a slight 

 anemia. In certain countries, the frequency 

 of A' in the gene pool follows expectation 

 for a recessive lethal gene. In other coun- 

 tries, however, A' is more frequent than 

 would be expected. How is this difference 



explained? It turns out that the heterozygote 

 AA' is more resistant than the AA homozy- 

 gote to certain kinds of malaria. Of course, 

 in nonmalarial countries, A' confers no 

 antimalarial advantage and so the fitness of 

 the heterozygote (1 — s) is lower than that of 

 the normal homozygote (1), while the A' A' 

 individual has a fitness of 0. As expected, 

 therefore, sickle cell anemia is rare or absent 

 in most of the world where certain forms of 

 malaria are absent. 



On the other hand, in certain malarial 

 countries, even though heterozygotes may 

 be slightly anemic, the advantage of being 

 resistant to malaria produces a greater over- 

 all fitness than does the AA genotype. Here 

 the fitness of the heterozygote, AA' , is 1 and 

 that of the normal homozygote, A A, is 1 — Si. 

 Mutant homozygotes. A' A' , have a fitness of 

 1 — So, where S2 equals 1, since all A' A' die 

 (even should A' A' be extremely resistant to 

 malaria). What natural selection does in 

 this case is to maintain both A and A' in the 

 gene pool. A' having a frequency equal to 



— ^ — This fraction can be read "the ad- 



Si + Si 



vantage of A' (as shown by the advantage of 

 AA' over AA) divided by the total disadvan- 

 tage of A and A'."' You see, then, that when 

 the heterozygote is more adaptive than either 

 homozygote, showing heterosis in this way, 

 natural selection will maintain a gene in the 

 gene pool even though it is lethal when 

 homozygous. 



It should be noted that while we have dis- 

 cussed heterosis in terms of the phenotypic 

 effects of the members of a pair of alleles, 

 this does not mean that the unit of heterotic 

 action is always a single pair of genes. Since 

 we have already seen, in Chapters 7 and 8, 

 that different pairs of genes interact in vari- 

 ous ways in producing phenotypes, it would 

 be no surprise to find that heterosis occurs as 

 the result of the effect of certain combina- 

 tions of nonalleles and alleles. 



It has been found, in D. pseudoohscura. 



