544 HEREDITY 
trasting characters must have separated as units, and in the 
separation no straggling part of a gene was left associated with 
the gene for the contrasting character. To this complete sep- 
aration of genes and consequently contrasting characters the 
term segregation was applied. 
The complete segregation ot characters also implies a purity 
of gametes. The constitution of an individual depends upon 
what the sperm and egg introduced into the fertilized egg from 
which the individual developed. Thus, if a plant is pure for 
tallness, the sperm and egg involved in the fertilization resulting 
in the production of this plant could not have contained genes 
for dwarfness. Of the two kinds of genes, they contained only 
shose for tallness. The same is true in case of dwarfness or any 
other one of a pair of contrasting characters. This really means 
that in a fertilization resulting in the production of a plant 
pure for a character both the sperm and egg have genes for the 
same contrasting character, and that an individual pure in re- 
spect to a character, therefore, is one that has inherited from 
both parents genes for the same character. In other words, a 
plant pure for a character is one that receives a double dose of 
genes for this character. On the other hand, plants, like the 
impure tall ones, have received genes for the dominant character 
from one parent and genes for the recessive character from the 
other parent, and hence they have only a single dose of genes 
for either of the characters. Such a plant we now speak of as 
being heterozygous, while plants having a double dose of genes 
and hence pure for a character are regarded as homozygous. 
Since plants pure for a character breed true, their gametes must 
be alike in respect to genes contained. The descendants of a 
homozygous parent propagated entirely by self-fertilization are 
of course pure and constitute what is known as a pure line. 
After tracing the behavior of single pairs of characters through 
successive generations, he took up the study of two or more pairs 
of contrasting characters, the aim being to determine how pairs 
of contrasting characters behave in respect to each other. For 
example, he crossed Peas characterized by smooth yellow seeds 
with Peas characterized by wrinkled green seeds. In this 
case he was dealing with two pairs of characters, smooth and 
wrinkled, and yellow and green, with smooth and yellow as 
dominants. He found that each pair of contrasting characters 
