236 GENETICS 



two cases only, namely, in 3 and 5, where the de- 

 fective parent is heterozygous, is there any chance 

 of unaffected offspring, and even in these cases the 

 defect is quite as likely to appear as not. It is ob- 

 vious that the only way to rid germplasm of a 

 dominant defect is by continued mating with 

 recessive individuals. By this method it is possible 

 in time to shake off the defect. When it once dis- 

 appears in any individual, it will never return unless 

 crossed back to a similar defective dominant strain. 



In other words, such a recessive extracted from a 

 heterozygous ancestry will breed just as true as a 

 recessive which was pure from the start. In both 

 instances there is an entire absence of the character 

 in question, and it is clear that this character can 

 thereafter never again reappear, since something 

 cannot be derived from nothing. 



On the other hand, if a defect is negative, depending 

 upon the absence of a normal dominant determiner, 

 as is usually the case with defects, it behaves as a 

 Mendelian recessive, that is, it is always apparent in 

 individuals developing from the homozygously de- 

 fective germplasm. 



It is certain, for example, that an imbecile which 

 has arisen from homozygous defective germplasm 

 carries only the determiner for imbecility in his own 

 germplasm, and when two such recessives mate, noth- 

 ing but imbecile offspring can result, for recessives 

 breed true. Nothing plus nothing equals nothing. 



An illustration of this principle is given in the fol- 

 lowing pedigree (Fig. 72) furnished by Goddard, 1910. 



