HEREDITY AND EVOLUTION 239 
emphasized by a consideration of the inheritance of two pairs 
of characters which Mendel studied in garden peas. 
In peas round seed is dominant over wrinkled seed, and 
_ yellow seed over green seed. When, therefore, we cross a plant 
having round yellow 
seed with one having 
wrinkled green seed, the 
F generation will have 
round yellow seed. If 
these hybrids are self- 
fertilized, they will give 
rise to the following 
kinds of plants in the 
following ratio: nine 
with round yellow seed, 
three with round green 
seed, three with wrin- 
kled yellow seed, and 
one with wrinkled green 
seed. This relationship 
is shown in Fig. 234, in 
which the small squares 
represent individuals, 
while circles represent 
gametes. #& stands for a 
factor for round; 7, for 
wrinkled; Y, for yel- 
low; and y, for green. 
From the diagram it 
will be seen that the 
F, generation contains a 

Fig. 234. Diagram showing inheritance of 
seed characters in garden peas 
Squares represent individuals; circles contain- 
ing letters are gametes ; letters represent factors 
as follows: R, round; r, wrinkled; Y, yellow; 
y, green 
factor for round, one for wrinkled, one for yellow, and one for 
green. When this plant forms gametes, both the male and the 
female gametes will show four different combinations of factors: 
namely, round and yellow, round and green, wrinkled and yel- 
low, and wrinkled and green. A male nucleus with the factors 
