VARIATION AND HEREDITY 



229 



gametes 







O 



O -' 



homozygous fusions to occur. A glance at the dia- 

 gram (fig. 83) will make this clear. 



One fourth would be pure recessive (DD), two 

 fourths impure dominant ((D) T) and (T (D)), 

 and one fourth pure 

 dominant (TT). 

 Now T (D) and (D) 

 T are identical and 

 contain the recessive 

 (D), which in a sub- 

 sequent generation 

 sorts out in a simi- 

 lar fashion, giving 

 homozygous dwarfs 

 and tails, and hetero- 

 zygous tails. The 

 last two classes 

 would be indistin- 

 guishable on account 

 of the dominance of 

 the tall character, 

 but would sort out 

 very differently in 

 continued breeding, 

 as experiment shows. 

 In Mendel's original experiment he got 787 dwarfs 

 and 277 tails in the first filial generation, and of the 

 latter 28 were pure in the succeeding generations. 

 In other words, the actual experimental result bore 

 out strongly the hypothesis formulated, which is 

 based upon the assumption of the purity of the 



O 



generation 



FIG. 83. Scheme of the possible 

 zygoses of the gametes of two parents that 

 differ with respect to one character, such as 

 the tallness and dwarfness of Mendel's 

 peas. The two parents breed true, that 

 is, have but one kind of gamete (homozy- 

 gous). There are four possible combina- 

 tions and hence four theoretical types of 

 zygotes. Owing to the phenomenon of 

 dominance, three of these cannot be out- 

 wardly distinguished from one another. 

 (Punnett.) 



