The Mendelian law is concerned with the dominance and recessiveness of plant and animal characters. It was clearly shown by Mendel, and later by Correns and de Vries, that, given a single plant with a character we wish to perpetuate, among a hundred of that individual's grandchildren, there can be secured twenty-five that will be counterparts of their unusual grandparent, so far as the one special character is concerned. This percentage is obtained by mating the prodigy with ordinary stock and excluding the resultant hybrids from being fertilized by any but other produce of the same original unusual individual. Another twenty-five per cent, will be equally as capable of reproducing the opposite character of the individual from which they sprang. The remaining fifty per cent, appear true to the type of their hybrid parents, but, like them, reveal their actual identity when their offspring follow the same unusual proportions.
The fact that such proportions can be relied upon added a new feeling of certainty and greatly encouraged attempts to perpetuate and multiply various features of plants. Of course, in the first generation, the prized character of the parent may be recessive or prevented from asserting itself by the more powerful opposite, and, until it was known that one-fourth of the next generation might return to the character in question, many attempts to breed in new features were abandoned after the apparent failure of the first cross. Instances of the operation of the same law were found in the animal kingdom.
Castle found that in guinea-pigs the extra length of hair was dominant over short hair which reappeared in Mendelian proportions in the succeeding generation. Albinism and smoothness of coat were also found to be inherited as recessive Mendelian characters. But of even greater interest than these unusual proportions is the exhibition of inheritance by unit characters. When we see length of hair being inherited independently of its color, and each of these independent of arrangement as to roughness or smoothness, we begin to realize the vast number of unit characters that go to make up an organism.
The unusual proportions, occurring so nearly accurately in large numbers, were highly interesting to the biologists and very suggestive to many persons not previously interested either in botany or zoology.
Mendel's law was of the greatest importance in pure science because any explanation of the fixed proportions must be based upon the nature of the gametes, and much new theory and research was undertaken to ascertain the basis of such seeming unnatural exactness of proportion.
If we assume that the gametes produced by hybrid individuals are pure to one or the other of the parental characters, the explanation of the Mendelian proportions is comparatively simple. Such assumption, however, is not justifiable in the light of our present knowledge. A