CONTRASTING CHARACTERS 545 



complete separation of genes and consequently contrasting 

 characters the term segr elation was applied. Since the characters 

 behave as units, that is, independently and not as a part of other 

 characters, they were called unit characters. 



The complete segregation of characters also implies a purity 

 of gametes. The constitution of an indivdual 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 the egg have genes for the same 

 contrasting character, and that an individual pure in respect 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 all 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. 



Two or more pairs of contrasting characters. After tracing 

 separately the behavior of single pairs of characters through 

 successive generations, he undertook to trace simultaneously the 

 behavior of two or more pairs of constrasting characters, the aim 

 being to determine how pairs of contrasting characters behave in 

 respect to each other. For example, he crossed Peas character- 

 ized 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 the contrasting 



