GENETIC VARIATIONS 317 



tlon by many students of genetics, and the results are of 

 much importance for the understanding of inheritance and 

 variation. There are two main classes of such genetic varia- 

 tions. One consists of changes in the number and grouping 

 of the chromosomes, the other of breakage and reunion of 

 the parts of the chromosomes. We consider these sepa- 

 rately. 



I. Changes in the Number and Grouping of the 



Chromosomes 



The commonest type of irregularity in the action of the 

 genetic system and that of which the consequences are best 

 known is that known as non-disjunction. This has been 

 described on earlier pages (page 59 and figure 20). In 

 rare cases, in forming germ cells, the two members of a 

 pair of chromosomes fail to separate. In consequence one 

 germ cell is formed which has two members of this pair, 

 while the other has no member of this pair. This gives rise 

 to many further irregularities in the individuals formed by 

 these germ cells. 



When the germ cell with two chromosomes of a pair 

 (figure 64, A) unites with one of the usual germ cells 

 having a single chromosome of that pair (B), a zygote is 

 formed that has three chromosomes for this pair instead 

 of the usual two (figure 64, C). Such zygotes are called 

 trisomies. The individuals developed from such trisomic 

 zygotes have the genes of this chromosome in threes in- 

 stead of in pairs. 



Also when the germ cell having no chromosome of a cer- 

 tain pair (figure 64, D) unites with a typical germ cell, a 

 zygote and individual is formed with only one chromosome 

 of that pair, the genes single instead of in pairs (figure 64, 

 F). 



Many cases in which such changes have occurred have 



