HERITAGE OF THE INDIVIDUAL 



279 



proportion of 9 yellow round to 3 yellow wrinkled to 3 green 

 round to 1 green wrinkled. (Fig. 140.) 



This logically can only be interpreted as indicating that 

 one of the original parent plants bore germ cells all contain- 

 ing the genes for yellow and for round peas (YR), while the 

 other parent plant bore cells 

 all containing the genes for 

 green and for wrinkled (yr) . 

 Such being the case, the re- 

 sulting zygote is YRyr, and 

 the hybrid which it forms 

 develops germ cells with all 

 the possible combinations 

 of these genes (except, of 

 course, Rr an.d Yy) which 

 are YR, Yr, yR, and yr. 

 Now, in turn, at fertiliza- 

 tion there are sixteen possi- 

 ble combinations of germ 

 cells, since there are four 

 different kinds of sperm and 

 four different kinds of eggs 

 with respect to the char- 

 acters in question. Accord- 

 ingly the F 2 generation, . 

 which is produced by the union of these gametes, is repre- 

 sented by one extracted dominant (YRYR), one extracted 

 recessive (yryr) , four (including the former two) homozygotes 

 and twelve heterozygotes. These sixteen individuals form 

 nine genotypes but, since only the dominant character is 

 expressed when dominant and recessive genes combine, they 

 are resolvable into four phenotypes (YR, Yr, yR, yr) in the 

 ratio 9 YR : 3 Yr : 3 yR : 1 yr. Thus the 9 : 3 : 3 : 1 



FIG. 142. Diagram classifying the six- 

 teen possible types of zygotes, shown in the 

 middle of Fig. 141, according to genotypes 

 (nine) and phenotypes (four) . (From Wal- 

 ter.) 



