THE APPLICATION TO MAN 



237 



The result is quite different, however, when one 

 parent only shows the defect. If the other parent is a 

 normal homozygote, as in case 4 of the accompanying 

 table, all the offspring will be normal in appearance, 

 but with the bar sinister of defectiveness in their 

 germplasm, while if the other parent is heterozygous 

 (Case 5), one half of the progeny will be defective. 

 Finally, when neither parent shows defectiveness 



EkO 



N 



c 



N]-r{N)@[r]®LN 



c 



[Nh<N)[rU^^ 



Fig. 72. — Pedigree chart illustrating the law that two defective parents 

 have only defective offspring. A, alcoholic ; C, criminalistic ; d, died ; 

 F, feeble-minded ; T, tubercular. After Goddard. 



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. 



