250 A TEXTBOOK OF GENERAL BOTANY 
tall peas with round yellow seeds and dwarf peas with wrinkled 
green seeds gives, in the /, generation, tall plants with round 
yellow seeds, as the characters tall, round, and yellow are domi- 
nant over the alternative characters. If it were desirable to have 
dwarf plants with round yellow seeds, these could be obtained 
in the F, generation. In this generation one out of sixty-four 
individuals would be homozygous for dwarf, round, and yellow. 
The progeny of these individuals would therefore breed true. 
The #, generation would also contain dwarf individuals with 
round yellow seeds which would not breed true. Such plants 
would be heterozygous for round or yellow or for both of these 
characters. In order to distinguish between the plants which 
are homozygous for the desired characters and those which are 
heterozygous it is only necessary to obtain a sufficient number 
of plants of the next generation and see which plants breed 
true. This example enables us to see how we can combine the 
desirable characters of one variety with those of another and get 
a variety that is superior to both of the parent varieties. 
Mendelism also shows that certain characters are the result 
of a heterozygous condition, and that we cannot obtain a variety 
that will breed true for such characters. Pink four-o’clocks 
afford a good example. As pink flowers are produced only by 
heterozygous individuals, pink-flowered plants will never breed 
true but will produce red-flowered, pink-flowered, and white- 
flowered plants in the ratio of one, two, and one. 
Mutations. It has been pointed out that the selection of con- 
tinuous variations does not result in hereditary changes, while 
hybridization may produce new varieties. There are, however, 
variations which are not the result of hybridization and which 
are hereditary. These may occur even in pure lines, that is, in 
strains descended from a single self-fertilized homozygous indi- 
vidual. Such hereditary variations are much less numerous than 
are fluctuating variations, and are known as mutations. Char- 
acters which appear as the result of mutations are inherited in 
Mendelian fashion and may be either dominant or recessive. In 
popular language, mutations are frequently spoken of as sports. 
