/ HEREDITY AND EVOLUTION 241 
Linkage. The number of chromosomes in an organism is usu- 
ally not very great, while, on the other hand, the organism has 
a large number of characters. It should follow from this that 
one chromosome would carry several or many different factors. 
Moreover, factors which are in the same chromosomes should 
be linked together in inheritance. This reasoning is borne out 
by the fact that many characters are linked together in inherit- 
ance. In the Chinese primrose the factors for red stigma, red 
flower color, long style, dark stem, and light corolla tube are 
linked together. In garden peas the factor for round seed as 
opposed to wrinkled seed and the factor for tendriled as opposed 
to nontendriled leaves go together. We may conclude, there- 
fore, that factors in the same chromosome are linked together, 
while those in different chromosomes are independent. 
Trihybrids. In the consideration of the inheritance of two 
pairs of characters we have seen that sixteen combinations are 
involved in the /, generation. These include plants of four 
different appearances and nine different germinal compositions. 
“When three pairs of characters are considered, sixty-four in- 
dividuals are involved in the F, generation. These include 
plants of eight different appearances and twenty-seven differ- 
ent germinal compositions. Only one of the sixty-four contains 
only dominant factors, and, likewise, only one contains only 
recessive characters. 
The inheritance of three pairs of characters may be illustrated 
by garden peas. In these plants we have seen that tallness is 
dominant over dwarfness, yellow seed is dominant over green 
seed, and round seed is dominant over wrinkled seed. If tall 
individuals with yellow round seed are crossed with dwart 
ones with green wrinkled seed, the offspring in the 7, genera- 
tion will contain factors for tallness, dwarfness, yellow, green, 
round, and wrinkled; but all the individuals will be tall with 
yellow round seeds, as these characters are dominant. 
When the F, generation forms gametes, these are of eight 
kinds, and the fusing of eight kinds of eggs with eight kinds 
of male nuclei gives rise to sixty-four combinations. These 
