120 



GENETICS AND EUGENICS 



Modification of the ratio, 9:3:3:1, due to linkage. When two Mendelian 

 characters are not wholly independent of each other, but show a tendency 

 to be inherited together, they are said to be coupled or linked to each other. 

 Thus, in the sweet pea, purple and red are alternative color forms, and 

 long pollen and short pollen are alternatives as to pollen shape. And if a 

 purple plant with long pollen is crossed with a red plant having round 

 pollen, four classes are obtained in r2, viz., purple long, purple round, 

 red long and red round. This being apparently a dihybrid Mendelian 



TABLE 10 



The F2 Ratio, 9:3:3:1, as Affected by Coupling or Linkage, A and B 

 Entering the Fi Zygote in the Same Gamete 



1 No coupling. 



2 Not distinguishable from the case in which A and B are due to a single genetic factor. 



cross, we should expect the four classes to be respectively as 9:3:3:1, but 

 in reality the classes purple long and red round (the parental combinations) 

 are in excess of these proportions. When these facts were discovered by 

 Bateson and Punnett, it was stated that coupling exists between the 

 characters purple and long and their allelomorphs red and round. Later, 

 however, when a cross was made between purple round and red long, it 

 was found that these combinations were in excess in r2. Purple and long 

 which in the first case were coupled, now showed repulsion. Morgan 

 explains both cases by supposing that the two character-pairs have deter- 

 miners or genes located near to each other in the germ-cell, probably in the 

 same chromosome, so that the parental combination has a tendency to 

 persist in F2. Morgan also proposes to substitute a single term, linkage, 

 for the two terms of Bateson, coupling and repulsion. 



