208 



CHAPTER 15 



nons. stillbirths, and infant deaths were 24 

 to 48 per cent higher when cousins married 

 than when parents were unrelated. Since, in 

 sonic cases, delects such as these are known 

 to be due to recessive genes in homozygous 

 condition, these results support the view that 

 homozygosis resulting from inbreeding can 

 produce detrimental effects. Although in- 

 breeding produces homozygosis and homo- 

 zygosis can lead to the appearance of de- 

 tects, it must not be inferred that inbreeding 

 is disadvantageous under all circumstances. 

 Many individuals do become homozygous 

 for detrimental genes as a result of inbreed- 

 ing, but just as many become homozygous 

 for the normal alleles. The success of self- 

 fertilizing species is testimony to the advan- 

 tage of homozygosity at least for some types 

 of organisms. 



* Heterosis 



In normally cross-fertilizing species, how- 

 ever, inbreeding usually results in a loss of 

 vigor which is directly linked to homozygosis. 

 What is the functional basis for the adaptive 

 superiority of heterozygotes. usually known 

 as heterosis or hybrid vigor? Consider the 

 three genotypic alternatives, A A, A A', A' A' 

 relative to their phenotypic effects. Suppose 

 A' A' is less vigorous than A A. Whether A 



is completelj or incompletely dominant to A' 



or shows no dominance to it. the A A' hetcr- 

 OZygOte will be superior to one of the 

 homozygotes. It is also possible that the 

 heterozygote has a greater adaptive value 

 than either type of homozygote. To illus- 

 trate this possibility, imagine that A is 

 pleiotropic. having a relatively great adaptive 

 effect with respect to trait X but a relatively 

 less adaptive effect with respect to trait Y. 

 whereas the reverse is true of A', namely, 

 relatively less adaptive for X and relatively 

 more adaptive for Y. In the event of no 

 dominance, the heterozygote is superior to 

 either homozygote. Heterosis can be pro- 

 duced, therefore, when the heterozygote is 

 superior to either one 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 by aa bbCC DD). 

 The F, (Aa bb CC Dd) is uniform yet more 

 vigorous (having normal alleles at three loci) 

 than either parent (each of which had nor- 

 mal alleles at two loci) because the domi- 

 nant alleles hide the detrimental effects of 

 the recessive ones. In this case the hetero- 

 zygous F 2 progeny carrying Aa bb CC Dd 

 are no more adaptive than the homozygotes, 

 AA bb CC DD. 



Frequency from Increase in Frequency Per cent 



Unrelated Parents with Cousin Marriage Increase 



CONGENITAL 

 MALFORMATION 



.011 



.005 



48 



STILLBIRTHS 



.025 



.006 



24 



INFANT DEATHS 



.023 



.008 



figure 15-5. Increased risk of genetic defect with cousin marriages. 

 Hiroshima and Nagasaki. ) 



34 



( Data from 



