98 



Outline of Genetics 



fruit fly, but a great deal of information is now being 

 accumulated on linkage in corn, and numerous scattered 

 demonstrations of the phenomenon have been made in 

 other organisms as well. Only a rather simple explana- 

 tion will be attempted here, to bring out merely some of 

 the fundamental principles of the phenomenon. 



When first considering Mendel's law, the state- 

 ment was made that more than one determiner might 

 be located on a given chromosome. As yet we have 



Dwarf White Parent 



4 Possible Gametes 



.-. F, Shows 9:3:3.1 Ratio 



Fig. 1 6. — Diagram showing normal dihybrid behavior when no 

 linkage is involved. 



considered no such case, but linkage involves exactly 

 this situation. In connection with some of Mendel's 

 original crosses fig. i6 will be recalled. In this case a 

 double dominant mates with a double recessive, and the 

 result is a dihybrid ratio in the F2 generation, following 

 the production of four types of gametes by the Fi plants. 

 Suppose, however, that the determiner T and the deter- 

 miner R are carried on the same chromosome, the situa- 

 tion would be as represented in fig. 17. Here the Fj 

 individuals produce only two types of gametes, as in a 

 monohybrid, so that the F2 presents what amounts to 



