GENETICS 



breeder definite expectations and a method of procedure. 



If, upon hybridization, the desired character be- - 

 haves like a recessive, then all that is necessary to\ 

 establish a pure stock exhibiting the character in ques- ( 

 tion, is to breed two recessives together, because reces- J 

 sives are always homozygous and, regardless of their / 

 ancestry, breed true. 



On the other hand, if the desired character proves , 

 to be a dominant, then it is necessary to determine 

 whether it is present in a duplex or a simplex condition ; j 

 in other words, whether it is homozygous or hetero- 

 zygous, for only homozygous organisms breed true. 

 Establishing a strain consists, consequently, in making 

 an organism homozygous. 



The test to determine whether a dominant character 

 is homozygous or heterozygous, that is, whether it 

 will breed true or not, can be made by a single cross 

 according to the procedure outlined in paragraph 8 

 of Chapter V. If, upon crossing the individual to be 

 tested with a recessive, it produces an entirely dominant 

 progeny, then its germplasm is duplex for this charac- 

 ter, and it will always reproduce the character in 

 either duplex or simplex condition according to what- 

 ever cross may be made with it. When crossed, for 

 instance, with another duplex dominant like itself, a 

 pure homozygous strain of the character in question 

 will be perpetuated. 



If, on the contrary, the dominant character to be 

 tested proves to be simplex or heterozygous, as de- 

 termined by the fact that, when crossed with a re- 

 cessive, 50 per cent of the progeny are recessive, then 



