HYBRIDIZING PLANTS 



135 



Of the 16 combinations in the table, some are genetical 

 dupHcates. Actually, there are nine different classes as fol- 

 lows: (1), (2, 5), (3, 9), (4, 7, 10, 13), (6), (8, 14), (11), 

 ( 12, 15 ) , and ( 16) . These different classes differ genetically 

 and are called genotypes. The genetical constitution of an 

 individual is its genotype. Actually there are only four 

 classes which can be distinguished by observation. These 

 are round yellow, wrinkled yellow, round green, and wrinkled 

 green. These four classes are phenotypes. The phenotype 

 of an individual is its appearance. The phenotype is deter- 

 mined by the interaction of the genotype and the environ- 

 ment. 



Mendel worked with seven pairs of characteristics. Actu- 

 ally there are hundreds of recognizable character differences 

 in garden peas. Most of these are conditioned by single gene 

 differences. This means that each of the seven chromosomes 

 has numerous genes. It can be shown genetically that these 

 many genes are arranged in linear order throughout the 

 length of the chromosome (Fig. 7). The important feature 

 of this situation is that the genes in the same chromosome 

 behave as though they were linked. The phenomenon is 

 called linkage. If a particular chromosome moves to a par- 

 ticular pole at meiosis, all of the genes which it contains move 

 as a unit to that pole. These genes are actually linked physi- 

 cally to one another. 



Linkage is seldom complete, however. When the homo- 

 logues pair during the first stages of meiosis, they come in 

 such close approximation that there is usually an exchange 

 of material from one chromosome to another. This exchange 

 of material is called crossing over. Crossing over normally 

 occurs whenever homologues pair normally. Since this ex- 

 change of material is regular, the only effect of linkage is to 

 reduce the size of the recombination classes. The amount 

 of linkage varies depending upon the position of the genes. 

 This is due to the fact that the crossing over occurs at ran- 



