[Chap. XXXIX MUTATIONS 481 



among the varieties of a species. The usual diploid (2n) number of 

 chromosomes in corn is 20. The number of chromosomes that have been 

 found in different varieties of corn are 21, 22, 23, 24, 25, 26, 27, and 28. 

 A third type of irregularity has been variously called "translocation," 

 "segmental interchange," and "crossing over." That is, when the pair of 

 homologous chromosomes are separating, they may be so interwoven 

 that a piece of one homolog remains attached to the other one. Many 

 such irresularities are known to occur, and thev have been studied with 

 interest as a means of discovering the relative locations of the genes 

 within the chromosome. The displaced piece of chromosome may have 

 certain genes whose presence can be detected by the appearance or 

 absence of certain characters in the plant. By comparing the effects of 

 different pieces of the same chromosome in different individual plants, 

 the relative position of the different genes in the chromosome may be 

 closely estimated. For the solution of such problems the combined efforts 

 of both cytologists and geneticists are needed. 



Irreeularities in reduction division in hvbrids mav be obtained (1) 

 by crossing remotely related plants having the same number of chromo- 

 somes, but chromosomes which previously have not been together in the 

 same cell, or (2) by crossing plants differing in chromosome number. 

 The irregularities that may occur in the pairing, or lack of pairing, of 

 the chromosomes in such hvbrids are rather numerous, and only a few 

 examples will be mentioned here. These irregularities of chromosomes, 

 though indirectly the result of cross-fertilization, should be distinguished 

 from the hybrid effects of cross-fertilization that are directly dependent 

 upon the mixing of chromosomes of two kinds of plants as described in 

 the preceding chapter. 



Of the eighteen recognized species of wheat, three have 7 pairs of 

 chromosomes, nine have 14 pairs, and six have 21 pairs. Self-fertile 

 hybrids are readily obtained from crosses between species having the 

 same number of chromosomes. Hybrids are less readily obtained by 

 crosses between species with different numbers of chromosomes, and 

 they are often self-sterile. Hybrids obtained by crossing species with 14 

 and 21 pairs of chromosomes are sometimes self-fertile. Hybrids between 

 species with 7 and 14 pairs of chromosomes or between those with 7 

 and 21 pairs are self-sterile; but they may be successfully back-crossed 

 with parents, or crossed with certain other species. Obviously several 

 new kinds of chromosome combinations may be obtained by such means. 



Hvbrids have also been obtained between species of different genera 



