Page:Popular Science Monthly Volume 80.djvu/379

Rh

We later found a plant of lenticularis showing a little red on the rays, and of course used this in a cross.

Could we predict the result of these crosses? Yes, to some extent. Could we regain the red as it was before the cross? Yes, no doubt, but in order to explain how, it is necessary to digress.

During the sixties, Gregor Mendel, Prälat at Brünn in Moravia, experimented with plants, especially garden peas. He was the first to appreciate the necessity of following up crosses for several successive generations, tabulating the results in each case, and ascertaining the numerical proportions of the differing forms resulting. He also took pains to consider the different sets of characters separately, treating them statistically as if they were different organisms. Working in this way, Mendel discovered that when two varieties are crossed the resulting hybrid is frequently not intermediate, but resembles one or the other parent. In other cases, when the hybrid, as a whole, seems intermediate, the several characters are nevertheless found to correspond with those of one or the other parent. When this sort of thing occurs, the character which comes uppermost in the cross is said to be dominant, the one which remains latent or hidden is called recessive. Inasmuch as fertilization results from the fusion of the germ-cells of the two parents, it is evident that each individual hybrid must contain material derived from both, although only the characters of one parent may be visible. Now Mendel found that when hybrids obtained as described were crossed together in the next generation he got, in simple cases, three of the "dominant" type to one of the "recessive." Of course the proportions would not be always thus, but whenever the number of cases was large they approximated so closely to the three-to-one ratio, that he became convinced that this was no accident. A simple theory was formulated, according to which the results arose from the chance combination of the elements in the germ cells. We may now make this clearer by a diagram in which D stands for the character which is dominant, E for that which is recessive.