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CHAPTER 15 



(a) is .01. In the population at equilib- 

 rium, therefore. AA:Aa:aa individuals have 

 frequencies oi 9801 10,000: 198 10,000: 

 I 10,000, respectively. Notice that Aa in- 

 dividuals are 1 C )S times more frequent than 

 aa, so that even if every aa did not repro- 

 duce, only one per cent of the a genes pre- 

 sent in the gene pool would be eliminated 

 each generation. This fact illustrates the 

 inefficiency of selection against homozygotes 

 for rare recessive genes, at least insofar as 

 lowering the frequency of such genes is con- 

 cerned. A A and Aa individuals apparently 

 marry at random but feebleminded people 

 do not. So panmixis does not occur with 

 respect to this trait, and persons with dif- 

 ferent genotypes tend to be restricted in their 

 marriages — all the available marriage part- 

 ners making up a person's reproductive 

 isolate. The occurrence of different repro- 

 ductive isolates for normals and phenyl- 

 ketonurics has little effect on the relative 

 frequencies of different genotypes in succes- 

 sive generations, because aa people have so 

 few of all the a genes present in the popula- 

 tion. Clearly, only marriages between two 

 A a individuals are of consequence, since 

 those are the major source of aa offspring. 



The example of phenylketonuria shows 

 that when a gene is rare and apparently com- 

 pletely recessive, nonrandom marriage has 

 little influence either upon gene frequency 

 or the diploid (heterozygous or homozygous) 

 genotypes in which it is found in the popula- 

 tion. When the mutant is relatively fre- 

 quent in the population, however, it is ob- 

 vious that nonrandom marriages raise the 

 frequencies of certain diploid genotypes and 

 lower others. Moreover, if there are adap- 

 tive differences for the different genotypes, 

 the composition of the gene pool can be 

 changed in a different direction or at a dif- 

 ferent frequency than would be predicted 

 for a population mating at random. 



Consider two ways in which mating can 



be nonrandom. The first invokes the tend- 

 ency of phenotypically similar individuals 

 (except for sex) to mate and is referred to 

 as assortive mating. This kind of breeding 

 pattern is generally true in animals including 

 human beings. The genetic result is the 

 production of more homozygotes than would 

 occur by randomly-chosen matings. 



The second departure from random mat- 

 ing involves inbreeding, the tendency for 

 mates to be more closely related in descent 

 than randomly chosen mates. What is the 

 effect of inbreeding carried out for a single 

 generation? This can be determined by 

 studying what happens to genes that are 

 heterozygous in the parent generation. 

 There are various degrees of inbreeding, the 

 closest form being self-fertilization. In self- 

 fertilization the heterozygote for a given pair 

 of genes, A a, produces progeny of which one 

 half are homozygous. In general, the de- 

 crease in heterozygosity because of self- 

 fertilization can be expressed as follows: 

 the chance that an offspring receives a given 

 gene in the male gamete is y 2 , and the 

 chance that it receives the same allele in 

 the female gamete is H; the chance that 

 the offspring is a homozygote for that allele, 

 therefore, is %. But there is an equal 

 chance that the offspring becomes homozy- 

 gous for the other allele, so that the total 

 chance for homozygosis from this type of 

 inbreeding is 50%. If all members of the 

 population are heterozygotes and self-fer- 

 tilize, then in each successive generation, 

 half of the genes that were heterozygous 

 become homozygous. 



Suppose, on the other hand, that a portion 

 of a population mating at random has X% 

 homozygous individuals. These could come 

 from matings between two heterozygotes, 

 two homozygotes, or a heterozygote and a 

 homozygote. If the gene pool is at equi- 

 librium, the random matings that tend to 

 increase homozygosis are counterbalanced 



