COMMON PRINCIPLES 



evident sex-linkage, are assumed to lie in one of the 23 pairs ot 

 autosomes. Of these a few are sex limited. Like most of the colour 

 genes of Lebistes (both sex-linked and autosomal) they are expressed 

 only in the male. Thus it seems that early baldness in men is the 

 expression of a heterozygote, which appears only in the homozygote 

 in women. A greater number show the more remarkable property 

 of incomplete sex limitation. They have greater penetrance in 

 the males. Cleft-palatc-with-hair-lip, for example, shows in a 

 higher proportion of homozygous males than females. Many 

 hereditary abnormaUties are determined by dominant genes, 



TABLE 28 



THE FREQUENCIES OF FIRST-COUSIN MARRIAGES IN 

 THE PARENTS OF AFFECTED INDIVIDUALS (HOG- 

 BEN, 1931; COCKAYNE, 1933; DAHLBERG, 1942) 



although in some the study is complicated by incomplete dominance 

 (or penetrance). 



Already in 1902 Garrod pointed out that the incidence of abnor- 

 malities and defects deteririined by recessive allelomorphs should 

 be higher in the progeny of first-cousin marriages than in the general 

 population. The frequency of cousin marriages in the parents of 

 homozygous segregants should increase with the increasing rarity 

 of the disease. If i in 10,000 of the population show the recessive 

 character, 5 per cent of their parents should be first cousins. If 

 I in 1,000,000, then the frequency should be 35 per cent. Estimates 

 have in fact been made, as given in Table 28, which correspond 

 rouglily with the frequency of the disease. 



Polygenic action gives a plausible genetic basis for the apparently 



347 



