310 GENETICS 



THE MEXDELIAX EXPECTATION- FOR DEFECTS 



likely to appear as not. It is obvious that the only 

 way to rid germplasm of a dominant defect is by con- 

 tinued mating with recessive individuals. By this 

 method it is possible in time to shake off the defect. 

 When it once disappears in any individual, it will never 

 return unless crossed back to a similar defective domi- 

 nant 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 in- 

 stances there is an entire absence of the character in 

 question, and it is clear that this character can there- 

 after 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 defective 

 germplasm. 



