CHAPTER XXX 

 LIxXKAGE PHENOMENA 



The Third Law. — Limitation of Independent Assort- 

 ment. In the last chapter it was shown that many char- 

 acter-pairs assort themselves independently of one an- 

 other. But since this independence is due to their 

 respective genes being carried in different chromosome- 

 pairs it is clear that there can be no more independent 

 pairs of characters than there are pairs of chromosomes. 

 For example, there are seven pairs of chromosomes in 

 peas. Mendel found just seven pairs of independent 

 characters, nor has the extensive work since his time 

 discovered any more. This fact is also true of all other 

 plants and animals so far studied. It may be stated as a 

 third law of heredity: The number of pairs of charac- 

 ters which can at the same time assort independently in 

 any organism is equal to the number of chromosome pairs 

 in its cells. 



The Fourth Law. — Characters whose genes are lo- 

 cated in the same chromosome are linked in heredity. 

 Both the third and fourth laws are obvious consequences 

 of the behavior of the chromosomes. Since these chromo- 

 somes are definite in number and maintain their individ- 

 uality, the total number of characters must be assorted 

 in an equal number of linked groups. That this is ac- 

 tually true is being abundantly proved by investigations 

 with both animals and plants. Groups of linked charac- 

 ters are already known in several plants, including the 

 sweetpea in which the phenomenon was originally dis- 

 covered. Linkage is also known in several animals, 

 including man. 



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