Chap. 20 THE PHYSICAL BASIS OF HERKDITY 413 



chromosomes in the sex cells, there are 8 and the fertilized egg has 16. 

 Thus the young plant starts with double the number of chromosomes and 

 larger cells than those of its parents. Giant tomatoes (Fig. 20.22) and 

 giant flowering marigolds- are polyploids. The radiant "Tetra Snaps" of cer- 

 tain seed catalogues are "Giant Tetraploid Snapdragons" that have giant 

 flowers and four times the usual haploid number of chromosomes. Animals 

 make a poor showing of polyploidy. It has been induced experimentally in 

 Drosophila, and in several species of salamanders chiefly by subjecting the 

 animals to low temperature (Fig. 20.23). 



Inbreeding and Outbreeding 



Inbreeding is the mating of near kin; cross breeding and outbreeding are 

 the matings of unrelated individuals. In many communities there is a great 

 deal of the former. Obviously, the more closely individuals are related, the 

 more hereditary traits they have in common; the better or worse are their 

 traits, and the better or worse for their descendants. Charles Darwin and 

 his wife Emma Wedgwood were first cousins, each with a long heritage of 

 desirable genes. In their case, nature and nurture joined in producing the 

 gifted and cultured Darwin family. Cleopatra was the descendant of six 

 generations of brother and sister marriages, yet the story of her life does not 

 imply that she was dull or helpless. 



Outbreeding usually produces individuals with unlike genes in which re- 



FiG. 20.22. Giant plants. Polyploidy in the tomato resulting from treatment with 

 colchicine, a, leaf and usual diploid number (12 pairs) of chromosomes; b, leaf 

 and chromosomes of triploid (3 sets of 12); c, leaf and chromosomes of tetraploid 

 (4 sets of 12). (After Jorgenson. Courtesy, Snyder: Principles of Heredity, ed. 4. 

 Boston, D. C. Heath and Co., 1951.) 



