452 



EVOLUTION, GENETICS, AND EUGENICS, 



The question arises with reference to this, as well as to many 

 other, dominant characters as to why, being dominant, the character 

 does not become more prevalent rather than remaining relatively rare. 

 The answer that seems most probable is that the brachydactylous hand 

 is very poorly adapted to human uses and those individuals possessing 

 it are seriously handicapped. Hence there may be marriage selection 

 against it. If it were more advantageous than the normal, doubtless 

 the incidence of the character would increase and it might in time be- 

 come the typical condition for the species. The same explanation 

 would apply equally well to all other cases of dominant abnormal 



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Fig. 82. — Inheritance of one form of cataract. Modified from Nettleship's 

 chart. The diagram reads thus: A man with cataract married a normal woman; 

 of their eight children six were affected with the disease. One of these married 

 an unaffected man; three of the children of this union were normal, sex unrecorded, 

 two defective. This same man married a second wife who was normal; their 

 eight children were all unaffected. So continue reading through five generations. 

 (From Downing.) 



Mendelian traits. It might also be asked why all the offspring of a 

 dominant parent do not show the character. The reason should be 

 obvious, namely, that most individuals are heterozygous for the char- 

 acter. A heterozygote mating with a recessive is expected to produce 

 one-half heterozygotes and one-half recessives, which is about what the 

 accompanying pedigree shows. 



Another case of a dominant character, not so definitely analyzed 

 as the last, is a type of hereditary cataract (Fig. 82). This condition is 

 due to an opaque region in the lens of the eye, which, when well ad- 

 vanced, may cause blindness. "In the particular form of the disease 

 here considered," says Downing, "it does not develop until middle life. 



