v] MENDELIAN HEREDITY 63 



short, the first four types of zygote, which all contain 

 both P and T, will be purple tails ; the next two 

 containing P but no T will be purple short ; the two 

 containing T but not P will be tall white, and the 

 last with neither P nor T will be short white. The 

 F2 offspring will thus appear in the ratio of 9 purple 

 tall, 3 purple short, 3 white tall, 1 white short. 

 Further, of the first group one will be homozygous 

 in both characters (PPTT), four homozygous in one 

 and heterozygous in the other (2PPTt, 2PpTT) and 

 four heterozygous in both (PpTt). Of the remainder, 

 one in each class will be homozygous in both char- 

 acters, and the others heterozygous in one, the 

 homozygous (pure) types being PPtt, ppTT and 

 pptt. 



It is clear then that by crossing two races which 

 differ in two allelomorphic characters, and self- 

 fertilising (or mating together) the crossed individuals, 

 in the F^ generation a definite proportion of neiv 

 pure combinations are produced. In the above 

 example, by crossing tall purple with short white, 

 in the second generation not only these types are 

 produced, but also short purple and tall white, and 

 by selecting the pure (homozygous) individuals pure 

 races of these new types are immediately established. 

 We thus obtain a new conception of organic char- 

 acters, as factors which can be replaced by alternative 

 characters without otherwise altering the constitution 



