HEREDITY AND EVOLUTION 239 
fertilizes an egg which has a factor for red, or vice versa, the 
fertilized egg has a single factor for white and a single factor for 
red, and the resulting plant will have pink flowers. When the 
F, hybrid gives rise to sexual nuclei, there will be, on the aver- 
age, an equal number of egg nuclei with the factor for red and 
egg nuclei with the factor for white. Likewise, there will be an 
equal number of pollen grains with the factor for red and pollen 
grains with the factor for white. The two kinds of male nuclei 
produced will, according to chance, fertilize an equal number of 
eggs. Moreover, each kind of male nucleus will tend to ferti- 
lize an equal number of eggs with the factor for red and eggs 
with the factor for white. The male nuclei with the factor for 
red will then fertilize eggs in the ratio of one with the factor 
for red to one with the factor for white, so that there will be 
one plant with red flowers to one with pink flowers. In a simi- 
lar way, the male nuclei with the factor for white will give rise 
to one plant with pink flowers to one plant with white flowers. 
Thus we get plants in the ratio of one plant with red flowers, 
two with pink, and one with white (Fig. 2382). 
Purity of gametes. Inheritance of color in the four-o’clock illus- 
trates one of the fundamental principles of Mendelian inherit- 
ance, that is, purity of gametes. The fusion of two gametes with 
alternative characters, such as red and white, results in a hybrid 
the body cells of which will contain the factors for both characters. 
When, however, this plant produces gametes, the factors for 
these two characters will separate, and one of them will be 
found in one gamete and the other in another gamete. One 
gamete will thus contain only one of two such alternative char- 
acters. In other words, a gamete of a four-o’clock can contain 
either a factor for red or a factor for white, but not both. In 
this way the alternative characters are segregated when gametes 
are formed, so that no gamete is a hybrid so far as a single pair 
of characters is concerned. The segregation of the factors is due 
to their being located in corresponding chromosomes which are 
separated and distributed to different nuclei during the reduc- 
tion division. 
