92 



Plant Genetics 



considered no such case, but linkage involves this situa- 

 tion. In connection with some of Mendel's original 

 crosses fig. 24 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. Suppose, 

 however, that determiner T and determiner 5 are on the 

 same chromosome, the situation would be as represented 



Dwarf Wrinkled 

 Parent 



4 Possible Gametes 

 .-. ¥, Shows 9 : 3 : 3 : 1 RaUo 



Fig. 24. — Diagram showing normal dihybrid behavior when no 

 linkage is involved. 



in fig. 25. This is linkage; that is, with two determiners 

 a monohybrid ratio instead of a dihybrid ratio is 

 obtained. Morgan's definition is as follows: "When 

 factors lie in different chromosomes they give the 

 Mendehan expectations; but when factors lie in the 

 same chromosome they may be said to be linked, and 

 they give departures from the Mendelian ratios." It 

 will be noted that this conception rests upon the belief 

 that the chromosome is the indivisible unit in inheritance, 

 a conception that seems, to have been justified by most 

 of the breeding results and which conforms to Mendelian 

 inheritance. 



