514 - Heredity and Evolution 



result if a nucleus fails to divide after the 

 chromosomes have divided, or if two daugh- 

 ter nuclei fuse before the cell divides. Such 

 an aberration occurring in a germ cell may 

 lead to the production of diploid (2m) gam- 

 etes. Such gametes give rise to triploid (3n) 

 offspring if fertilized by normal (n) gametes; 

 or to tetraploid (4?;) offspring if fertilized by 

 other diploid gametes. 



Both triploid and tetraploid individuals 

 have been found in many species (Fig. 26-29). 

 Tetraploids particularly are apt to be larger 

 than their diploid prototypes; and a number 

 of other phenotypic differences may accom- 

 pany the aberration. 



As to the production of gametes, triploid 

 and tetraploid individuals differ quite mark- 

 edly. Usually all the gametes of a tetraploid 

 are diploid, and all are capable of taking 



part in fertilization. But the gametes of a 

 triploid individual vary as to their chromo- 

 some count, all the way from n to In, with a 

 majority falling in the intermediate range. 

 And since only the n and 2h gametes can 

 participate successfully in fertilization, tri- 

 ploid individuals are highly sterile. 



In nature many tetraploid stocks are found 

 among plants, but very few animal tetra- 

 ploids occur. Among cross-fertilizing species 

 there is little chance that both parents should 

 simultaneously produce aberrational (2?;) 

 gametes; and unless this occurs the aberra- 

 tion is almost certain to be eliminated in the 

 next generation, due to the sterility of the 

 triploid offspring. But among self-pollinating 

 plants, a single aberration may reach both 

 the male and female gametes, and thus tetra- 

 ploidy has a fair chance of becoming estab- 



FLOWERS 



FRUITS 



NORMAL 



Fig. 26-29. Flowers, seed capsules, and chromosome groups of haploid, 

 diploid, triploid, and tetraploid Jimson weeds {Datura stramonium). The 

 small size of the seed capsules of the haploid and triploid plants is due to 

 the low fertility of these forms, producing relatively few seeds. (Adapted 

 from Blakeslee.) 



