10 THE THEORY OF THE GENE 



are separated in the hybrid; for yellow and round that 

 went in together have, in some cases, come out separated. 

 Similarly for green and wrinkled. 



Mendel also showed that when three, and even four, 

 pairs of characters enter a cross their elements are inde- 

 pendently assorted in the germ-cells of the hybrid. 



It might, then, have seemed justifiable to extend this 

 conclusion to as many pairs of characters as enter any 

 particular cross. This would mean that there are as many 

 independent pairs of elements in the germinal material 

 as there are possible characters. Subsequent work has 

 shown, however, that Mendel 's second law of independent 

 assortment has a more restricted application, since many 

 pairs of elements do not assort freely, but certain ele- 

 ments that enter together show a tendency to remain 

 together in succeeding generations. This is called linkage. 



Linkage. 



Mendel's paper was recovered in 1900. Four years 

 later Bateson and Punnett reported observations that 

 did not give the numerical results expected for two inde- 

 pendent pairs of characters. For instance, when a sweet 

 pea having purple flower-color and long pollen grains is 

 crossed to one with red flowers and round pollen grains, 

 the two types that go in together come out together more 

 frequently than expected for independent assortment of 

 purple-red and round-long (Fig. 9). They spoke of these 

 results as due to repulsion between the combinations 

 purple and long and red and round, that came from oppo- 

 site parents. Today these relations are called linkage. By 

 linkage we mean that when certain characters enter a 

 cross together, they tend to remain together in later 

 generations, or, stated in a negative way, certain pairs 

 of characters do not assort at random. 



It would seem, then, so far as linkage holds, that there 



