448 EVOLUTION, GENETICS, AND EUGENICS 



they follow the familiar mode of heredity described for sex-linked 

 characters in Drosophila. Dominants may fairly readily be distin- 

 guished by the fact that at least one of the parents, one grandparent, 

 one great-grandparent, etc., exhibit the character; a dominant charac- 

 ter appears in every generation. Ordinary autosomic, or non-sex- 

 linked, recessives are characterized by the irregularity of their in- 

 cidence in pedigrees. They may skip one or several generations and 

 only appear at all when the same recessive gene is present in the germ- 

 plasm of two mating individuals, in which case it will appear in the 

 following ratios according to the genetic make-up of the parents: (a) 

 If both are heterozygous, three out of four offspring will show the domi- 

 nant and one the recessive character; (b) if one is a heterozygote and 

 one a recessive, the offspring will be half heterozygous dominants and 

 half recessives. Thus we see that recessive characters may be hidden 

 for a long time awaiting a favorable mating to give them expression. 



Most of the best pedigrees of human characters deal with relatively 

 rare and atypical or abnormal characters; in fact, most of them deal 

 with what we ordinarily call freaks or pathological conditions. The 

 reason for this is that rare and unusual conditions are much more 

 easily detected and recognized. Ordinary, common differences are 

 hard to detect and hard to distinguish, and therefore difficult to follow 

 in pedigrees. 



It is not our intention in this chapter to attempt an exhaustive 

 treatment of the facts of heredity ascertained by the pedigree method. 

 We shall merely list some of the commonest cases of hereditary char- 

 acters and give a few pedigrees illustrating them. 



The following is a list of human unit characters classed as domi- 

 nants, ordinary recessives, and sex-linked recessives: 



A. DOMINANT CHARACTERS 



Skin and Hair: 



Dark skin Dominant over blond or albino 



(probably due to two or more 

 pairs of genes) 



Spotted with white Dominant over uniformly colored 



Tylosis and ichthyosis Thickened or scaly skin 



Epidermolysis Excessive blisters of skin 



Brown or black hair Dominant over red and flaxen (prob- 

 ably due to more than one 

 gene) 



Red hair Dominant over flaxen 



Beaded hair Single hairs are not uniform in diam- 

 eter 



