Mendelian genetics

“ But this is beside the mark, for modern genetics shows very positively that adaptive characters are inherited in exactly the same way as are those that are not adaptive; and I have already pointed out that we cannot study a single mutant factor without at the same time studying one of the factors responsible for normal characters, for the two together constitute the Mendelian pair. ”

“ 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. ”

“ In the examination of single family histories we should remember that a single pedigree may not accurately illustrate a general law of heredity—again, an individual case may belong to a group of cases without representing them fairly. Even in observing illustrations of Mendel's laws allowance has to be made for the variability due to "chance" meetings of germ cells. It is only when large numbers of individuals are observed that the typical Mendelian fractions and ratios can be strictly observed. ”

“ It is ordinarily resistant to "contamination" by other factors with which it may come in contact in the cell. The first fundamental principle of Mendelism, then, is the existence of relatively constant units, the Mendelian factors, as the basis for transmission of all the traits that go to make up an animal or plant.
Experimental breeding gives reason to believe that each factor has one or more alternatives, which may take its place in the mechanism of heredity, thereby changing the visible character of the individual plant or animal in which it occurs. ”

“ As a fact, it is extremely rare, being found in only a few families. The similar trait of orthodactyly or symphalangism, which likewise appears to be a good Mendelian dominant, seems to exist in only one family. Traits like these, which are easily defined and occur very rarely, make up a large part of the cases of probably Mendelian heredity. They are little more than curiosities, their rarity and abnormal nature depriving them of evolutionary significance other than to demonstrate that Mendelian heredity does operate in man. ”

“ It is obvious that these are the conditions which are fulfilled in a simple Mendelian case, and there is nothing in this pedigree to contradict the assertion that brachydactyly, whatever it may be due to, behaves as a simple dominant to the normal form, i.e. that it depends upon a factor which the normal does not contain. The recessive normals cannot transmit the affected condition whatever their ancestry. Once free they are always free, and can marry other normals with full confidence that none of their children will show the deformity. ”

“ Such forms cannot, of course, ever be bred true, and where several factors are concerned they may when bred together produce but a small proportion of offspring like themselves. As soon, however, as their constitution has been analysed and expressed in terms of Mendelian factors, pure strains can be built up which when crossed will give nothing but offspring of the desired heterozygous form. ”

“ It is by no means certain, however, that all inheritance is Mendelian, for there still remains much evidence that the hereditary basis of certain characters may be resident in the cytoplasm, rather than in the chromosomes. A recent book by Morgan, Sturtevant, Müller and Bridges (1915) , entitled “the mechanism of Mendelian heredity” gives the cytological explanation of Mendelian inheritance.
Americans have from the first taken a leading part in this field of research and have been quick to recognize its practical applications to the improvement of breeds in both animals and plants. ”

“ The Mendelian laws find verification in man. Every race, whether a sub-species or a variety, has an hereditary possession of certain characters; a possession which is completely transmitted to the descendants, in whom is preserved the same germ plasm as in the progenitors.
The researches of C. B. and G. Davenport seem to have proved the recessive character of albinism and its obedience to the Mendelian law. Hurst has presented figures which show that the inheritance of colour in the iris of the human eye obeys Mendelian laws. ”

“ The female-producing sperms will each contain a white X and the male-producing sperms will each contain an indifferent Y chromosome. Chance meetings of egg and sperm will give the four F2 classes (bottom row) . These consist of white eyed and red eyed females and white eyed and red eyed males. The ratio here is 1:1 and not three to one (3:1) as in other Mendelian cases. But Mendel's law of segregation is not transgressed, as the preceding analysis has shown; for, the chromosomes have followed strictly the course laid down on Mendel's principle for the distribution of factors. ”

“ If however instead of pure extreme varieties we were to take a pair of varieties differing normally by only a foot or two, we might, owing to the masking effects of conditions, &c., have great difficulty in distinguishing the three forms in the second generation. There would besides be twice as many heterozygous individuals as homozygous individuals of each kind, giving a symmetrical distribution of heights, and who might not—in pre-Mendelian days—have accepted such evidence—made still less clear by influence of conditions—as proof of Continuous Variation both of zygotes and gametes? ”

“ These are the phenomena upon which Mendel touches in his brief paper on Hieracium. As he there states, the hybrids, if they are fertile at all, produce offspring like themselves, not like their parents. In further illustration of this phenomenon he cites Wichura’s Salix hybrids. Perhaps some dozen other such illustrations could be given which rest on good evidence. To these cases the Mendelian principle will in nowise apply, nor is it easy to conceive any modification of the law of ancestral heredity which can express them. ”

“ There is a tendency among Mendelians and mutationists to overestimate the importance of experiments in comparison with reasoning, either inductive or deductive. Bateson, however, has admitted that Mendelian experiments and observations on mutation have not solved the problem of adaptation. ”

“ Will natural selection really serve to explain all? Must all the various characters of {3}plants and animals be supposed to owe their existence to the gradual operation of this factor working upon small variations?
Of recent years there has arisen a school of biologists to whom the terms mutationist and Mendelian are frequently applied. Influenced by the writings of Bateson and de Vries, and by the experimental results that have flowed from Mendel's discovery in heredity, they have come to regard the process of evolution as a discontinuous one. ”

“ On account of the fact of variation every individual is in a certain sense a hybrid. One's two parents have the species characters in common but there are certain distinctive traits that hybridize and follow Mendel's law of heredity. By no means is it to be understood that all individual distinctive traits follow this rule in heredity. Many individual characteristics are what we have learned to call fluctuations—small deviations above or below an average condition of a group. Such differences play no part in Mendelian heredity. ”

“ At the outset we should say that the knowledge of human heredity is as yet largely of the statistical sort. We know how a great many characters are inherited, on the average. The subject of Mendelian heredity is so new that there has been hardly time to investigate more than a few human characteristics from this point of view. Certain conditions add to the difficulties here. First, many, probably most, of the more important human traits are complexes, not units, and it is a long and difficult process to analyze them into their units, with which alone Mendelism deals. ”

“ It is unnecessary to point out that if all AB’s are to look alike, both the varieties A and B must be pure—not in the common sense of descended, as far as can be traced, through individuals identical with themselves, but pure in the Mendelian sense, that is to say that each must be at that moment producing only homogeneous gametes bearing the same characters A and B respectively. Purity of pedigree in the breeder’s sense is a distinct matter altogether. ”

“ How can we say that a complex trait is or is not inherited according to some form of Mendel's law when we do not know the nature of the units of which it is composed? We can make no statements about the Mendelian inheritance of such a trait until it is factored into its units. A considerable number of human characteristics are really known to be heritable according to this formula, enough so that several general rules of human heredity have been formulated. ”

“ We may therefore use the statistical formulas of heredity as a kind of temporary expedient, enabling us to make statements regarding inheritance of certain characters in the group or class, pending the time when we shall be able to give the facts a more precise and more "final" expression in Mendelian formulas. Many human traits are indeed already known to Mendelize. Most of these are, however, "abnormal" traits or pathological conditions; we are still in the dark regarding the actually Mendelian or non-Mendelian inheritance of most of man's normal characteristics. ”

“ For the Mendelian principle of heredity asserts a proposition absolutely at variance with all the laws of ancestral heredity, however formulated. In those cases to which it applies strictly, this principle declares that the cross-breeding of parents need not diminish the purity of their germ-cells or consequently the purity of their offspring. When in such cases individuals bearing opposite characters, A and B, are crossed, the germ-cells of the resulting cross-bred, AB, are each to be bearers either of character A or of character B, not both. ”

“ Mendel's experiments afford further evidence of the same fact, in that the organisms themselves are found to have means of dissolving such alliances and thus of holding to the paths on which their varietal divergencies have gone forward. The theory that hybridization assists evolution by encouraging variability is shown to have a distinct limit, since little evolutionary progress would come from mere combination of the stable or divergent characters which are a prerequisite of the Mendelian experiments. ”

“ This consists in collecting all the evidence possible, arranging it in the form of pedigrees, and comparing it with standard cases already worked out in animals and plants. In this way it has been possible to demonstrate in man the existence of several characters showing simple Mendelian inheritance. As few besides medical men have hitherto been concerned practically with heredity, such records as exist are, for the most part, records of deformity or of disease. ”

“ Mating of two unaffected persons, even though each had one affected parent, would be fit because the offspring would all be unaffected, barring the possibility of a new variation or mutation to this character, which would be extremely unlikely. Such a pedigree as this illustrates very well how a knowledge of Mendelian heredity may be of the greatest value practically, in determining the fitness or unfitness of marriages in families where an abnormality or defect is known to occur. ”

“ For it is the essence of Mendelism as of Weismannism that not only sex but all other congenital characters are determined in the fertilised ovum or zygote. The meaning of a recessive character in Mendelian terminology is one that is hidden by a dominant character, and both of them are due to factors in the gametes, particularly in the chromosomes of the gametes which come together in fertilisation. For example, in fowls rose comb is dominant over single. A dominant is something present which is absent in the recessive: the rose comb is due to a factor which is absent from the single. ”

“ It is not inconceivable that definite differences in chemical composition render the protoplasm of one species toxic to the gametes of the other, and if this is so it is not impossible that we may some day be able to express these differences in terms of Mendelian factors. The very nature of the case makes it one of extreme difficulty for experimental investigation. At any rate, we realise more clearly than before that the problem of species is not one that can be resolved by the study of morphology or of systematics. ”

“ When only a few easily recognized characters are concerned, it has been possible to trace a certain regularity, conforming in general to the Mendelian law. But different species differ not in one way, but in a thousand ways. These traits are so interwoven and so variously related, some of them cumulative and some contradictory, that in most cases no law determining which characters shall be dominant in each case and which recessive can be made out. While the Mendelian law may be of universal application, it is in a few cases only that its facts can be clearly seen. ”