THE APPLICATION TO MAN 



311 



It is certain, for example, that an imbecile which has 

 arisen from homozygous defective germplasm carries 

 only the determiner for imbecility in his own germ- 

 plasm, and when two such recessives mate, nothing but 

 imbecile offspring can result, for recessives breed true. 

 Nothing plus nothing equals nothing. 



For practical purposes it is unimportant to know 



FIG. 92. Pedigree chart illustrating the law that two defective 

 parents have only defective offspring. A, alcoholic; (7, 

 criminalistic ; d, died; F f feeble-minded; T, tubercular. 

 After Goddard. 



whether or not feeble-mindedness, or any similar defect, 

 is Mendelian in behavior. The fact that it is heredi- 

 tary is enough. 



An illustration of this principle is given in the 

 above pedigree (Fig. 92) furnished by Goddard, 1910. 

 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 it is heterozygous (Case 5), one half of the 

 progeny will be defective. 



