MENDEL'S FIRST LAW 23 



tion of the characters themselves, it seems better, I think, 

 to avoid as far as possible this application of the word. 

 The factor for tall and the factor for short are said to be 

 allelomorphio to each other. The parents are generally 

 designated by Pi ; the first hybrid generation is known 

 as the first filial generation, or briefly F^. The next 

 generation, derived from F^ is called F2, etc. When one 

 member of the pair of contrasted characters appears in 

 Fi to the exclusion of the other it is said to be dominant, 

 the eclipsed character is said to be recessive. The hybrid 

 itself is said to be heterozygous, meaning that it contains 

 one factor or gene of each kind, while an individual con- 

 taining both genes of the same sort is said to be homo- 

 zygous for the genes involved. Mendel did not emphasize 

 the idea that even in pure races each character is also 

 represented, as a rule, by a pair of factors or genes that 

 segregate in the formation of the germ-cells in the same 

 way as do the pair of contrasted genes in the hetero- 

 zygotes, but at the present time this idea is accepted by 

 all geneticists. It was at least implied on MendePs view 

 that the two pure classes in F^ {SS and ss), formed by 

 the recombination of two like genes, are identical with 

 the two grandparental races (Pi). 



A crucial test of the correctness of the assumption that 

 segregation of the members of a pair of elements takes 

 place in the germ-cells of the hybrid, consists in back- 

 crossing the hybrid (Pi) to one of the parent stock, viz., 

 to the not dominant stock, here the short pea. Since short 

 is recessive to tall, it will not influence the height of the 

 offspring when a tall and a short factor are brought 

 together. Such a cross should show whether the germ- 

 cells of the hybrid are, as postulated, of two sorts, and 

 whether equal numbers of each sort are produced. Mendel 

 made such tests, and obtained equal numbers of two kinds 



of offspring. 



Mendel obtained results like these with tall versus 

 short peas for other pairs of characters, such as f asciated 

 versus normal stems, hard versus soft pod, yellow versus 



