Thomas Hunt Morgan

“ If one can control the matings, so that out-breeding takes place each time, the result would undoubtedly be like that in our diagram; but what chance is there for a recessive character, that is neither beneficial nor injurious, if left to itself, to contaminate widely the race with its gene? The answer is that for any one defect there is hardly any chance at all. On the other hand, there is always a possibility that a defect may become widespread despite the chances against each in turn. ”

“ It is true that the student of Mendelian heredity does not often trouble himself about the nature of the character that he studies. He is concerned rather with its mode of inheritance. But the geneticist knows that opposed to each defect-producing element [2] in the germ-plasm there is a normal partner of that element which we call its allelomorph. We can not study the inheritance of one member of such a pair of genes without at the same time studying the other. ”

“ Of course if a dominant character is advantageous in itself, it will have a better chance of spreading through the race, than will an advantageous recessive character, because every hybrid that carries one dominant gene shows also the character, which increases the chance that it will propagate and spread the genes. ”

“ There is a growing impression that a good deal of feeblemindedness and insanity are environmental rather than hereditary traits; poverty, malnutrition, and especially syphilis are said to play a considerable rôle in their production. It is unsafe therefore to conclude that the human germ-plasm is as badly contaminated as some pessimists seem to think.
If we turn now more directly to special kinds of human inheritance we shall find a great deal of evidence showing that the same laws of inheritance that hold for animals and for plants apply to man. ”

“ We do not know how widespread recessive genes are in the human germ-plasm. The fact that defective individuals appear in certain communities may be safely interpreted to mean that individuals bearing the same gene have at last come together. On the other hand, the absence of such individuals from the community, at large, may only mean that the chance of suitable combinations is small, and does not mean necessarily that the gene in question is confined to the community within which the defects have been recorded. ”

“ Curiously enough no one has as yet had the temerity to suggest that some of the high-grade imbecile types—the moron, for example—might represent an ancestral stage of the human race. If this were true, intelligence would then be looked upon as an innovation in the race, that has not yet spread to all of its members. ”

“ While in the great majority of cases, the scheme of color-blindness is that shown by the diagram, we know that occasionally the machinery may be changed to give a somewhat different result. It is possible, for example, that a color-blind man married to a perfectly normal woman may rarely produce a color-blind son. A few years ago such a result would have appeared to upset the entire scheme of sex-linked inheritance, today we understand how such cases may arise through a process that is called non-disjunction, which is best illustrated by numerous cases well worked out in Drosophila. ”

“ If a fly with vestigial wings, a recessive character, is crossed to a wild fly with long wings, all the offspring (F1’s) will have long wings. If these are bred to each other the offspring will be of two kinds, like their grandparents, in the ratio of three long winged to one vestigial fly. The extracted vestigials will breed true to vestigial. The fact that the gene for vestigial has been carried by long winged F1 parents has not affected the gene in any way, for the second generation of vestigials has wings as short as those of their grandparents. ”

“ I propose now to pass in review some characters in man known to be inherited, choosing preferably those that come nearest to the field of pathology, or belonging to it. I shall begin with comparatively simple cases, about which there can be little doubt, and pass to more and more difficult situations. ”

“ In some combinations the hybrid may be more vigorous and more fertile than either parental race. Hence it may have a better chance of survival than an individual of either parent stock. It is a difficult question, that we cannot answer at present, whether a mixed strain has a better chance of survival than one or another of the strains of which it is made up. The possibility that some hybrid strains may be better than either pure strain is enough to put one on his guard against the popular doctrine of racial purity so-called. ”

“ Nevertheless the sex-ratio in the inbred stock continues as before, and since the missing males are those with red eyes we know that the tumor-gene is still present and doing its deadly work—only now the young male larvæ die even before they reach the age at which the tumor is due to appear.
So far I have spoken of heredity as though that term had become synonymous with Mendelian heredity. Those of as who are at work on Mendelian inheritance are often criticized as too narrow. It is said that we do not recognize that any other kind of inheritance takes place. ”

“ The number of offspring in a family is usually too small to serve as a sample of the germ-plasm of the parents. (2) Therefore, since recourse must be had to many families for sufficient data, it is essential that the diagnosis of the defects of the parents and of the children is correct. A single mistake may throw the result into confusion. In cases where the defect is structural, a correct classification may be possible, but in other cases, especially where psychological defects are involved, the diagnosis is difficult and the results, in consequence, less certain. ”

“ The linkage of yellow and eosin enables one to pick out in each generation those daughters that carry the tumor-factor.
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All males that get their single X with this tumor-gene will die; therefore, since no adult males carry it, normal males must be used for mating in each generation. They are mated to females that are heterozygous for the chromosome carrying the tumor genes. Such matings as I have said always give two daughters to one son. But since half the daughters are normal and half carry the gene for tumor it is desirable to be able to pick out the latter from the stock. ”

“ If the human germ-plasm is continually mutating to produce one or another kind of specific defect, this will increase the chance for any recurrent defect to finally establish itself. That particular mutations do recur in other animals is now abundantly established by evidence that comes from several sources. ”

“ The malformations that reappear in successive generations are general rather than specific. Such organs as the eye are those hardest hit, but this is supposed to be rather a by-product of the general debility of the individual. Stockard points out that the alcohol has affected the germ cells, and it is through these that the effects are transmitted. Now if one or more genes had been permanently changed we should expect to have evidence of Mendelian inheritance. The results do not show convincingly that the inheritance is not Mendelian, but it does not appear to be so. ”

“ Therefore we have made use of a trick we call “marking the chromosome,” which means that we use a male whose sex chromosome carries a known gene near the tumor locus. By using this type of male in successive generations we get two types of daughters: one type like their surviving brothers in eye color that do not carry the tumor-gene and the other daughter with normal eyes that carries it. ”

“ In the next generation several degrees of resistance were noted—and a few plants were even more susceptible than their grandparents.
It is interesting again to note that susceptibility and immunity are species and variety characters in these cases, but this does not mean that the differences are not Mendelian. It suggests however the possibility that several or many factor differences are often involved.
There is no more interesting field in which genetics and pathology meet than that of cancer. ”

“ One of the most remarkable cases of heredity in man is found in the so-called blood groups. As first definitely shown by Von Dungern and Hirschfeld in 1910, the inheritance of the four blood groups conforms to Mendel’s laws. ”

“ A female with the tumor-factor, normal wings and red eyes, in one of her sex-chromosomes and with the factors for yellow wings and eosin eyes in the other is bred in each generation to a male with yellow wings and eosin eyes. In the next generation there are twice as many daughters as sons, since all the sons that carry the black chromosome die. ”

“ Often the best that we can do in the case of man is to try to find the simplest Mendelian formula to which the evidence will fit. If one factor-difference will not suffice, then two must be tried; if two will not do, then three must be tried, etc. ”

“ Hemophilia is also sex-linked in inheritance. It is known to be much more common in men than in women, the explanation for this is the same as in the other case. In affected individuals the blood fails to coagulate quickly and the difference in chemical composition of the blood is, in contrast to normal, the inherited character.
Fig. 8. Representing the kinds of individuals expected when an individual of the blood group type AaBb marries individual of the same blood type, namely AaBb. Sixteen kinds of individuals are possible in the ratio of 9:3:3:1. ”

“ In such an imaginary situation, the ratio of cancer-developing mice may not appear to correspond to any of the known Mendelian ratios, because superimposed on the genetic situation there would be added results depending on the frequency of mutation when a specific gene is present. ”

“ In the F2 generation the frequency for the extracted double dominant will be that of the original strain, that of the F2 heterozygotes will be the same as that of the F1, and the extracted double recessive class will not develop cancer at all. ”

“ I emphasize the statement that certain mixtures of races may have a biological advantage. It is equally possible that other combinations may have a biological disadvantage. We are far from being able to state at present what combinations are beneficial and what are biologically injurious. ”

“ Whatever advantages some kinds of pure races of mankind may have, from a political, religious or militaristic viewpoint, this should not blind us to the possibility of the biological advantages that certain mixtures may bring about. ”

“ It is an interesting problem, one of deep significance I think for the future of the human race, but mixed up as it is at present with difficult social and political questions it is a problem that only a light-hearted amateur or a politician is likely to be dogmatic about. ”

“ Inspection of the diagram will show that group II (with serum bb) precipitates III and IV, and group III (with serum aa) precipitates II and IV. Further the serum of group I (aa bb) precipitates all of the other groups; while the serum of group IV precipitates none of the others.
My fourth illustration has probably in some cases a glandular basis, and in this sense has probably also a quantitative chemical background. Height or stature in man is, in part, an hereditary trait. ”

“ In connection with the question of spreading of mutant genes in the race there is another consideration, seldom referred to, that may occasionally have some weight in accounting for the dispersal of genes. ”

“ Color-blind women are rare, because they can never arise unless a color-blind man marries a woman who is color-blind, or else marries a normal woman who had a color-blind father, or had a mother heterozygous for color-blindness, Fig. ”

“ Larger numbers of tests will be necessary before it is possible to state how many factors are needed. A curious feature of the case should not pass unnoticed. Many or all of the factors for susceptibility must be assumed to be dominant. ”

“ There is a race of fowls known as Campines in which there are two kinds of males, hen-feathered males and cock-feathered males. If the hen-feathered male is castrated, the new feathers that develop are the long feathers of the cock-feathered male, Fig. ”

“ These same luteal cells are like those present in the stroma of the ovary of all female birds. If we assume that they make an internal secretion that prevents the development of cock-feathering, both in the normal hen and in hen-feathered cocks, we have a complete explanation of all the facts. ”