I \ 



158 GENETICS 



recessives 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 condi- 

 tion ; in other words, whether it is homozygous or 

 heterozygous, 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 para- 

 graph 8 of Chapter , VII. If, upon crossing the 

 individual to be tested with a recessive, it produces 

 an entirely dominant progeny, then its germp 



duplex for this character, and it will always reproduce 

 the character in either duplex or simplex condition 

 according to what it may be crossed with. 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 

 it requires more than a single generation to establish a 

 homozygous dominant strain. 



In random inbreeding of diverse strains if the re- 

 cessives are constantly eliminated as they appear, a 

 population is gradually obtained which is composed 



