Mendel found that there were certain peculiarities in plants which he termed “unit characters” that were transmitted from parent to offspring in a definite way. His classical work was on the propagation of the ordinary garden pea, in which case he found that a quality like tallness, as contrasted with dwarfness, was transmitted as follows:—
If tall and dwarf peas were crossed, he found in the first generation nothing but tall peas. But if these peas were allowed to grow and fertilize themselves, in the next generation he got tall and dwarf peas in the ratio of three to one. The dwarf peas in this case bred true, i.e. when they were planted by themselves and self-fertilized there was never anything but dwarf peas, no matter how many generations were tested. On the other hand, the tall peas were divisible by experiment into two groups; first, those that always bred true, viz. always tall peas; and secondly, another group that bred tall and dwarf in the same ratio of three to one; and from these the same cycle was repeated. Mendel called the character, which did not appear in the first generation (dwarfness), “recessive”; the other (tallness) he called “dominant.” The recessive factor is now generally considered to be due to the absence of something which, if present, would give the dominant factor. According to this view, dwarfness is simply the absence of tallness.
This law has been found to hold true for many unit characters in many plants and animals. Since study in human heredity has been taken up, it has been a natural question, Does this same law apply to human beings? It has been found that it does apply in the case of many qualities, like color of hair, albinism, brachydactylism, and other peculiarities. Investigation has of late been extended to mental conditions. Rosanoff has shown pretty clearly that the law applies in the case of insanity, while Davenport and Weeks have shown evidence that it applies in cases of epilepsy.
Our own studies lead us to believe that it also applies in the case of feeble-mindedness, but this will be taken up in a later work to which we have already referred. We do not know that feeble-mindedness is a “unit character.” Indeed, there are many reasons for thinking that it cannot be. But assuming for the sake of simplifying our illustration that it is a “unit character,” then we have something like the following conditions.
If two feeble-minded people marry, then we have the same unit character in both, and all of the offspring will be feeble-minded; and if these offspring select feeble-minded mates, then the same thing will continue. But what will happen if a feeble-minded person takes a normal mate? If feeble-mindedness is recessive (due to the absence of something that would make for normality), we would expect in the first generation from such a union all normal children, and if these children marry persons like themselves, i.e. the offspring of one normal and one defective parent, then the offspring would be normal and defective in the ratio of three to one. Of the normal children, one third would breed true and we would have a normal line of descent.
Without following the illustration further, we see already that it is questionable whether we ought to say that the original feeble-minded individual should have been sterilized because he was feeble-minded. We see that in the first generation all of his children were normal and in the next generation one fourth of them were normal and bred true. We should not forget, however, that one fourth of his grandchildren would be feeble-minded and that two other fourths had the power of begetting feeble-minded children. We must not forget, either, that these are averages, and that for the full carrying out of these figures there must be a large enough number of offspring to give the law of averages room to have full play. In other words, any marriage which, according to the Mendelian principle, would give normals and defectives in the ratio of three to one might result in only one child. That child might happen to be one of the feeble-minded ones, and so there is propagated nothing but the feeble-minded type. It is equally true that it might be the normal child, with a consequent normal line of descendants; or still again, it might be one of the intermediate ones that are capable of reproducing again the ratio of three normal to one defective, so that the chance is only one in four of such offspring starting a normal line.
Let us now turn to the facts as we have them in the Kallikak family. The only offspring from Martin Kallikak Sr. and the nameless feeble-minded girl was a son who proved to be feeble-minded. He married a normal woman and had five feeble-minded children and two normal ones. This is in accordance with Mendelian expectation; that is to say, there should have been part normal and part defective, half and half, if there had been children enough to give the law of averages a chance to assert itself. The question, then, comes right there. Should Martin Jr. have been sterilized? We would thus have saved five feeble-minded individuals and their horrible progeny, but we would also have deprived society of two normal individuals; and, as the results show, these two normals married normal people and became the first of a series of generations of normal people.
Taking this family as a whole, we have the following figures:—