THE 

 POPULAR SCIENCE 



» 



MONTHLY. 





3>, 



JANUARY, 1914 



THE MECHANISM OP HEREDITY AS INDICATED BY THE 

 INHERITANCE OF LINKED CHARACTERS 



By Peofessor T. H. MORGAN, 



COLUMBIA UNIVERSITY 



IT is generally recognized to-day that the central idea of Mendel's 

 discovery in regard to heredity is that when two contrasting ele- 

 ments enter a hybrid, one from each parent, they separate in the germ- 

 cells of the hybrid, so that the germ-cells are pure like those of the orig- 

 inal parents in regard to each element. 1 Chance meetings of the germ- 

 cells give the ratios that are characteristic of Mendelian heredity. This 

 is illustrated by the example that Mendel gave. 



When a pea having green seeds is crossed with a pea having yellow 

 seeds a hybrid pea is produced. When the germ-cells of the hybrid are 

 ripened, each ovule carries either the element for green or that for 

 yellow, but never both. Yellow and green have separated. The same 

 separation occurs in the formation of the pollen. If self-fertilization 

 now takes place chance combinations of the yellow- or green-bearing 

 ovules with the yellow- or green-bearing pollen give one pure yellow 

 pea, two hybrid (yellow) peas, one pure green pea, as shown in Fig. 1. 

 Mendel discovered that the same principle holds when two pairs of 



i Mendel speaks of characters as forming pairs. To-day we speak of fac- 

 tors or genes as the paired, elements (allelomorphs) in the germ-cells, and these 

 are supposed to act as differentials in producing the characters in the adult ani- 

 mal or plant. The English school considers the presence of a factor as one 

 allelomorph and its absence as the contrasting factor. For instance, if yellow 

 color is due to a present factor then if it were lost the color that results is 

 green. But since we know nothing about the material in the germ-plasm that 

 by interacting with other parts gives yellow in one case and green in the other, 

 it seems to me gratuitous to postulate the nature of the change in the germ 

 plasma. It is only necessary to assume that the original factor and a new 

 factor form a pair without in any way committing one's self as to how these 

 two allelomorphic factors are related to each other. 



