380 LINKAGE PHENOMENA 



5. The fifth principle is that the crossing-over or ex- 

 change of corresponding parts of homologous chromo- 

 somes causes the breaks in linkage of characters. 



6. The sixth principle asserts that by comparing the 

 percentages of crossing-over it is possible to show that 

 the genes are arranged in the chromosomes in linear order. 



7. Since sex in higher animals is determined by the 

 chromosomes, characters whose genes are located in the 

 sex chromosomes show sex linkage. 



