Mendelian inheritance

“ 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. ”

“ We may see, as it were afar off, facts that will be clearer to the scientific men of fifty years hence, facts of the Mendelian inheritance of physical characters, leading, on the one hand, to the maintenance of types very little changed even through thousands of years, and on the other to the combination of diverse heritages from many sources, making an individual in many cases a mosaic of characteristics from different ancestral types. ”

“ The pedigree is thus, in respect of eye-colour—and of other characters also which are not here described—clearly Mendelian in its manifestations. It shows that the offspring of two very different types of human races exhibit the same mode of Mendelian inheritance as do the descendants of two contrasted parents of the same race. ”

“ Very careful inquiry would be needed before such a possibility could be negatived. For example, we know that the Laws based on Ancestry can apply to alternative inheritance; witness the case of the Basset-hounds. Here there is no simple Mendelian dominance; but are we sure116 there is no purity of germ-cells? The new conception goes a long way and it may well reach to such facts as these.
But for the present we will assume that Mendel’s principle applies only to certain phenomena of alternative inheritance, which is as far as our warrant yet runs. ”

“ 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. ”

“ 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. ”

“ The now famous Mendelian Law of Inheritance, first discovered in 1865 by Mendel, an Augustinian monk and Abbott of Brünn, and completely ignored till the year 1900, when it was rediscovered by De Vries and others, is also strongly confirmatory of Weismann’s analysis of the principle of heredity. ”

“ If there are cases in which such segregation does not take place they belong to other forms of inheritance than the Mendelian: if segregation occurs in every instance there is no other type of inheritance than that discovered by Mendel. Are there cases which do not segregate according to Mendelian expectation?
Fig. 64. Diagram of Inheritance of Color-blindness through Female. A color-blind female transmits her defect to all her sons, to half of her granddaughters and to half of her grandsons. Corresponding distribution of sex chromosomes on right. ”

“ 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 putative father was not the real father.
(3) The Mendelian theory of inheritance is wrong.
The Mendelian theory is established on so firm a basis that, in the absence of more numerous exceptions, (3) may be rejected. There is no reason for supposing that the observations were inaccurate, and we are therefore brought to the conclusion that in such a case the child is illegitimate.
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Fig. ”

“ The family had resided in England for several generations, so that in this case there was no question of a further admixture of black. Most noticeable is the family produced by a very dark lady who had married a white man. Some of the children were intermediate in colour, but two were fair whites and two were dark as dark Hindus. This sharp segregation or splitting out of blacks and whites in addition to intermediates strongly suggests that the nature of the inheritance is Mendelian, though it may be complicated by the existence of several factors which may also react upon one another. ”

“ It is not, however, in the economic field, important as this may be, that Mendel’s discovery is likely to have most meaning for us: rather it is in the new light in which man will come to view himself and his fellow creatures. To-day we are almost entirely ignorant of the unit-characters that go to make the difference between one man and another. A few diseases, such as alcaptonuria and congenital cataract, a digital malformation, and probably eye colour, are as yet the only cases in which inheritance has been shown to run upon Mendelian lines. ”

“ The Mendelian theory of inheritance in general has been subjected to a prolonged and widely ramifying series of tests, and it seems in the present state of knowledge to present a satisfactory and consistent explanation of the facts. ”

“ 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. ”

“ And the bearing of these facts on the other problem of the transmission of acquired characters is quite obvious. Mendelism provides no place for any such transmission. Mendel's Law is sometimes called the law of alternative inheritance, thus embodying in its name the thought that offspring may show the characters possessed by one parent or by the other, but that it cannot develop any characters whatever which were not manifest or latent in the ancestry. Changes in the environment during the embryonic stage, it is true, seem sometimes to be registered in the growing form ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ If rose and single comb chickens are crossed, rose combs are dominant. The first generation will all have rose combs. The second generation will have one-fourth single combs that will breed true, one-fourth rose combs that will breed rose combs only, and one-half that again will give all three types.
Mendel's Law works all right in cases where pure unit characteristics are to be found. For the great practical problems in inheritance, Mendel's law is utterly hopeless. ”

“ 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. ”

“ The laws of heredity, particularly those discovered by Mendel, have been tested to such an extent as to make them of positive moment to human life. The eugenics idea, started in England by Sir Francis Galton, aims at a practical application of the known principles of inheritance to the good of the human race. ”

“ 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. ”

“ I regard it as very important that this real meaning of Mendel’s principle should be most clearly understood.
From the fact of the splitting of hybrids in the second generation most important consequences may be drawn for the theory of inheritance; the split individuals, if crossed with each other, always give an offspring which remains pure; there is no further splitting and no other change whatever. The germ-cells produced by the split individuals of the second generation may therefore be said to be “pure,” as pure as were those of the grandparents. ”

“ Suppose, then, that instead of two pure types, we had six or eight breeding together, each pair forming their own heterozygote, there would be a very remote chance of such purity or fixity of type whether of gamete or zygote being detected.
Dominance, as we have seen, is merely a phenomenon incidental to specific cases, between which no other common property has yet been perceived. In the phenomena of blended inheritance we clearly have no dominance. In the cases of alternative inheritance studied by Galton and Pearson there is evidently no universal dominance. ”

“ Meanwhile, a great body of workers was studying with great minuteness the material basis of heredity, the sexual elements. Their investigations disclosed in the cell a complete basis for just this kind of alternative inheritance and led up to the rediscovery of Mendel's law simultaneously by several different experimental breeders, foremost among whom was the Dutch botanist, de Vries. Mendel found that in cross-breeding between alternative characters, one uniformly dominates in the offspring from its very nature, while the other disappears. ”

“ Since heredity lies at the basis of the breeder's work, it is evident that any contribution to a more exact knowledge of this subject must prove of service to him, and there is no doubt that he will be able to profit by Mendelian knowledge in the conduct of his operations. Indeed, as we shall see later, these ideas have already led to striking results in the raising of new and more profitable varieties. In the first place, heredity is a question of individuals. Identity of appearance is no sure guide to reproductive qualities. ”

“ According to Galton's celebrated law of ancestral inheritance, the qualities of any organism are determined to the extent of a certain fraction by its two parents taken together as a "mid-parent," that a smaller definite fraction is contributed by the grandparents taken together as a mid-grandparent, and so on to earlier generations. But Mendel's Law has far greater definiteness, it explains more accurately the cases of alternative inheritance, and it may be shown to hold for blended and mosaic inheritance as well. ”