332 GENETICS 



two pieces, which may be passed on separately to members 

 of the next generation. Thus, if the two pieces are both 

 passed on, certain genes that in earHer generations were 

 Hnked, are in later generations not linked. 



Often however one piece of a broken chromosome is lost. 

 This Is because In cell division each chromosome has at- 

 tached to it at a certain point (usually either at one end or 

 at the middle) a single spindle fiber, which holds it In place. 

 When a chromosome Is broken, one of the pieces is without 

 the spindle fiber attachment, so that it may get lost. In order 

 that such a piece shall be carried through the cell divisions 

 in the usual way, it must become attached to another piece 

 that has a spindle fiber attachment. 



When a piece of a chromosome is entirely lost, carrying 

 with it of course a number of genes, the organism is unable 

 to develop, unless it contains also another chromosome of 

 that pair in which these genes are not missing. As we saw on 

 page 194, complete deficiency for even a single gene usually 

 prevents development. 



In translocations, there is of course a change in the 

 method of inheritance. If a piece of an autosome is thus 

 transferred to an X-chromosome, the characteristics de- 

 pendent on the genes in the transferred piece are in later 

 generations no longer inherited in the autosomal or typical 

 Mendelian manner, but according to the rules of sex-linked 

 inheritance. Conversely, if a piece from X is translocated to 

 an autosome, the inheritance of certain sex-linked characters 

 is changed to the autosomal type. Many cases of this kind 

 are now known. They demonstrate in a striking way the 

 dependence of the method of inheritance on the location of 

 the gene in a particular chromosome; when the location is 

 changed the method of inheritance changes correspondingly. 



When the order of the genes is reversed in a certain part 

 of the chromosome, as indicated in figure 67, C, this of 

 course changes the relation of genes to each other. Certain 



