Page:Popular Science Monthly Volume 62.djvu/282

276 in related varieties, where the remaining characters of both varieties are unimportant. Hence, in practice, we have in reality to deal with dihybrids in many cases. It should also be remembered that, if we treat a hybrid as a dihybrid, neglecting all but the two characters of most concern, the type we select actually splits up into fixed types with reference to all other characters, so that in a few generations we can secure uniformity, even in minor characters, by selection.

There is a very interesting phase of the subject which, for the sake of clearness, has been purposely overlooked in what has been said above. We have dealt only with the case in which a parent character appears in the hybrid in a fully developed state, or is not apparent at all. This is actually the case with the characters discussed above. Cases are known, however, in which both of a pair of opposite characters appear in the hybrid. This may result in a form intermediate between the parents, as I found to be the case when I crossed short-headed club wheats with the ordinary long-headed varieties. The same phenomenon appeared in crosses between varieties with red chaff and those with white chaff. Sometimes, in crosses between white and red flowers, for instance, the heterozygote types are variegated. It is easy to see that this fact may have an important bearing on the flavor and other characters of hybrid fruits, such as apples, peaches, strawberries, etc. It is highly probable that a great majority of these fruits are heterozygote in character, which fact would explain their well-known variability when grown from seed. It would naturally be expected, since flavors are due to the presence, in various proportions, of certain chemical substances, that entirely new flavors should be found in seedlings of this character, for in almost every seedling we should have a new combination of the flavor-giving substances.

One reason why Mendel's law was not discovered long ago is doubtless to be found in the fact that the large majority of seedlings that have come under the breeder's eye have had heterozygote parents of unknown constitution. If all our leading commercial varieties had been commonly close-fertilized, the law would long ago have forced itself upon us. Professor Bailey's remarkable and careful work on hybrid squashes and pumpkins probably came to naught for this very reason. Had he done the same work with varieties that are normally close-fertilized, he would probably have discovered this law. He was on the right road, but he was in the wrong vehicle.

Let us consider what results would follow the growing of apple seed generation after generation with close-fertilization, if the characters of the apple obey Mendel's law. We start with a tree that is already multihybrid. Suppose it to consist of N pairs of opposite character, ''A-A′, B-B′,. . . ,N-N′.'' The hybrid and the first