312 GENETICS 



Finally, when neither parent shows defectiveness but 

 one carries the defect as a heterozygote (Case 7), then 

 there will be no defective children, while if both parents 

 are heterozygous there is one chance in four that the 

 offspring will be defective. 



As a matter of fact, defectives usually mate with 

 defectives for the simple reason that normals ordi- 

 narily avoid them, so it comes about that streams of 

 poor germplasm naturally flowing together tend to the 

 inbreeding of like defects. 



Davenport * lays down the following general eugenic 

 rules for the guidance of those who would produce 

 offspring wisely: "If the negative character is, as in 

 polydactylism and night-blindness, the normal char- 

 acter, then normals should marry normals, and they 

 may be even cousins. If the negative character is 

 abnormal, as imbecility and liability to respiratory 

 diseases, then the marriage of two abnormals means 

 probably all children abnormal; the marriage of two 

 normals from defective strains means about one quar- 

 ter of the children abnormal; but the marriage of a 

 normal of the defective strain with one of a normal 

 strain will probably lead to strong children. The 

 worst possible marriage in this class of cases is that 

 of cousins from the defective strain, especially if one 

 or both have the defect. In a word, the consanguineous 

 marriage of persons one or both of whom have the same 

 undesirable defect, is highly unfit, and the marriage 

 of even unrelated persons who both belong to strains 



1 Davenport. Rep. of Amer. Breeder*' A*oc., Vol. VI, p. 431, 

 1910. 



