GAMETES OF TALL PURPLE HYBRID 361 



one has the gene P and the other the gene p. This sort of 

 difference in the chromosome pairs is characteristic of the 

 nuclei of all hybrids. Always one or more chromosome 

 pairs have at least one gene in one chromosome different 

 from the corresponding one in its mate. 



The Gametes of the Tall Purple Hybrid. — The re- 

 duction divisions in the tall white and short purple par- 

 ents produce only one kind of gamete for each because the 

 members of each chromosome pair are exactly alike. In 

 the Tall Purple hybrid more than one kind of gamete is 

 formed, because in the segregation of its two chromosome 

 pairs different genes go to the opposite poles of the 

 spindle. Furthermore, it has been noted in previous chap- 

 ters that it is apparently wholly a matter of chance which 

 way a chromosome pair lies on the nuclear spindle, and 

 therefore also a matter of chance to which pole either of 

 its members goes. The diagram shows the two ways in 





Fig. 101 is a diagram of two waj-^s of arranging chromosomes of 

 the tall purple hybrid on the spindle. 



which these two chromosome pairs may be arranged on 

 the spindle of the reduction division of the hybrid in 

 reference to one another. 



Position I produces two kinds of gametes TF (Tall 

 Purple) and ty (short white). Position II similarly pro- 

 duces the gametes Tp (Tall white) and tF (short 

 Purple). Since one position on the spindle is just as 

 likely as the other, it follows that four kinds of gametes 

 are produced in equal number. 



The Combination of the Four Kinds of Gametes. — 

 Since the hybrid can produce four kinds of sperms and 

 four kinds of eggs, it is apparent that it can give rise to 

 as many kinds of descendants as there are possible com- 

 binations of gametes. The diagram (called a Punnett 



