544 HEREDITY 



trasting characters must have separated as units, and in the 

 separation no straggHng 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 

 those 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, hke 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-fertihzation 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 



