MENDELIAN INHERITANCE 223 



may unite with a yellow-bearing ovule, and the result is a 

 heterozygous yellow, since yellow is dominant. Or a yellow- 

 bearing anther may unite with a yellow-bearing ovule, giving a 

 homozygous yellow; or finally a yellow-bearing anther may 

 unite with a green-bearing ovule, giving a heterozygous yellow. 

 Thus there will be three yellows to one green or one pure 

 yellow, two heterozygous yellows and one pure green (Fig. 94, 

 F2 generation). 



Mendel found further, that these pure greens, if continually 

 self-fertilized, never gave rise to yellow peas, and that the pure 



PARENTS 



FIG. 96. Diagram to illustrate the history of the gametes of crossed white and 

 red Mirabilis. A gamete with factor for white and one with factor for red unite 

 to form the pink zygote of FI. The gametes in FI are homozygous for red or 

 white, and these, by random mating, give the Mendelian ratio. (From Morgan.) 



yellows never gave rise to green peas, while the mixed yellows 

 and greens, on self-fertilization, always produced offspring in 

 the proportion of three yellow to one green. 



A similar result is obtained with white and red races of 

 Mirabilis jalapa, the "four o'clock," in which the hybrid (F), is 

 pink (Figs. 95 and 96). Here both allelomorphs for color take 

 part in the hybrid flower, forming a composite pink (Fi) which, 



