566 GENETICS AND EVOLUTION 



crossing over ol the first and second, and the second and third. These 

 facts are best exphiined by the assumption that the genes lie in a 

 linear order in the chromosome. 



279. Chromosome Maps 



All the genes in a particular chromosome constitute a linkage 

 group. In all the species tested the number of linkage groups deter- 

 mined by genetic tests and the number of pairs of chromosomes ob- 

 served under the microscope are the same. This is another bit of 

 evidence that the genes are located in the chromosomes and not else- 

 where within the cell. The genes which make up a linkage group re- 

 main constant from generation to generation and are altered only by 

 some major change in chromosome morphology such as a translocation 

 (p. 685), in which a piece of one chromosome breaks off and becomes 

 attached to a different, nonhomologous chromosome. The linkage be- 

 tween two particular genes, such as the linkage between tall and 

 spherical in tomatoes, is called a specific linkage. The specific linkage 



lOi 



r\ 



Ga.nxeT;iz-S : 



Single crossovers 



Gametes ' 



DouLle crossovers 

 Figure 32.7. Diagram illustrating crossing over, the exchange of segments of 



chromosomes during synapsis. See text for discussion. 



