482 Orgamsvis Are Products of Heredity mid Enviromiient unit ix 



t 



U 



'■Py 



k 



a' 



r-x ■' 





■^M 



P'lG, 433. The evening primrose (left) and one of its iiuitants (gigas or giant). What 

 do you notice about the flowers and the leaves of the mutant? (b. m. davis) 



in chromosome numbers. What kind of 

 irregularity would double the chromo- 

 some number? See Exercise 3. 



Normally a plant or animal has tMO 

 genes that affect some character. If in- 

 stead of two genes there are three or 

 four genes, or only one, the character is 

 often changed. Recently such changes in 

 chromosome number have been arti- 

 ficially produced by chemical means. 

 Colchicine can be used to cause doubling 

 of the number of chromosomes in some 

 plants. 



Other chromosome changes. Still other 

 irregularities occur in chromosomes. 

 Sometimes a piece of a chromosome 

 breaks off and attaches itself to the other 

 chromosome of the pair. When reduc- 

 tion division takes place one gamete gets 

 the chromosome with the added piece 

 and will have two genes for some charac- 

 ters; the other will have the short chro- 

 mosome and will lack certain genes. New 



individuals produced from such gametes 

 are likely to have new characters. 



De Vries and mutations. The term mu- 

 tation was first used about 1901 by a 

 Dutch botanist, Hugo De \"ries (184S- 

 19:55), as a result of his experiments with 

 the evening primrose {Oenothera La- 

 vmrckimia). By controlHng the matings 

 and keeping careful notes De Vries was 

 able to show that new t>'pes of plants 

 appeared and that some of the new types 

 bred true, that is, produced offspring 

 w'xxh the same new character. Among his 

 plants appeared some with red-veined 

 leaves, some \\'\x\\ broader leaves, some 

 that were dwarfed, and some that were 

 giants. Some of the changes were striking 

 and others were scarcely noticeable but 

 they bred true. 



Dc Vries said that changes took place 

 in the germ plasm and that these changes 

 were inherited. He used the word mu- 

 tation, to mean a change in the germ 



