GENETIC FACTORS IN THE ORIGIN OF DIVERSITY 399 



17.13). Such increase in ciiromosome number by addition of one or more 

 single chromosomes is called aneuploidy; it seems to have been important 

 in producing varieties of plants. 



Suppose that both pairs of chromatids in the secondary oocyte of 

 Fig. 17.3 failed to separate. As a result the second polar body would be 



Before 



FIG. 17.12. Reciprocal translocation of entire right "arms" of two nonhomolo- 

 gous chromosomes. 



empty of chromosomes, the ovum containing two long chromatids and 

 two short ones. Such an ovum would be diploid, instead of haploid, 

 containing as many chromosomes as the oogonium and the other cells of 

 the body contain. When such a diploid ovum is fertilized by a normal 

 haploid sperm the resulting fertilized ovum has three of each kind of 



TRISOMIC 



FIG. 17.13. Production of a trisomic fertilized ovum (zygote). Nondisjunction of 

 "short" chromosomes (see Fig. 17.3) resulted in an ovum containing two of them. 

 This ovum was fertilized by a normal sperm cell. 



chromosome, and is called a triploid (Fig. 17.14). Diploid sperm cells may 

 also arise by suppression of normal meiosis. When a diploid ovum is 

 fertilized by a diploid sperm cell a tetraploid is the result (Fig. 17.14). 

 Increase in chromosome number by addition of complete haploid sets is 

 called polyploidy. In our diagrams a haploid set has consisted of two 

 chromosomes, one long one and one short one. The actual number varies 



