Genet

“ The genet, no more than the mule, can reproduce. If one or the other is desired, of necessity recourse is had to the two species. In extremely rare cases, fecundity remains among some of their descendants, but it diminishes gradually from the second generation down to the third, fourth, and fifth. The same result is shown in the union of the canary bird. I could here accumulate a crowd of analogous details. Above all, two great general facts appear that comprehend all, and are the expression of the law ”

“ 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. ”

“ If I have appeared at times overcritical concerning the application of genetics to pathology, it is not because I do not sympathize with the attempts that have been made to apply genetics to pathology. I realize, of course, that from the nature of the case much of this work is pioneer work, where rough and ready methods have often to be resorted to. So long as this is kept in view, no harm can be done in attempting to find how far Mendel’s principles can apply to heredity in man. ”

“ The Mendelians take seeds of known origin, and plant them and note the result. They carry out experimental breeding not only amongst plants but amongst the higher animals, including mammals who, in all essentials of structure and function, are one with ourselves. It is not possible, I believe, to over-estimate the supreme importance of Mendelian enquiry for eugenics. Eugenics is founded upon heredity, and genetics, which is Professor Bateson's name for the physiology of heredity and variation, is now working at [307] the very heart of those natural phenomena upon which eugenics depends. ”

“ Even where the mutational variation appears to be one of gain, as happens when a positive character appears de novo in the phenotype, or when a dilute parental character is intensified in the offspring, it is, nevertheless, interpretable as a result of germinal loss, the loss, namely, total or partial, of a genetic inhibitor. Such inhibitive genes or factors are known to exist. ”

“ Problems in heredity and evolution have taken on a new interest since the importance and validity of Mendel’s discovery have been recognized. To meet this development of biology the journal Genetics was begun in 1916 for the publication of technical papers, while the Journal of Heredity, modified from the American Breeders Magazine, is devoted to popular articles on animal and plant breeding, and Eugenics.
On the whole, the science of zoology is now assuming a closer relation to practical affairs. ”

“ If the number of ancestral germ-plasms received from the parents were reduced to half in the ovary of the young animal, how then could parthenogenetic development ever take place? It is true that we cannot at once assert the impossibility of an early ‘reducing division’ on this account, for as I have shown above, the power to develope parthenogenetically depends upon the quantity of germ-plasm contained in the mature egg; the necessary amount might be produced by growth, quite independently of the number of different kinds of ancestral germ-plasms which form its constituents. ”

“ It is often too readily concluded that characters which appear constantly in successive generations are congenital; many of them are doubtless acquired afresh by each generation owing to the fact that the external conditions remain constant. It has not yet become possible to separate with certainty those characters which are genetic from those which are acquired. ”

“ Jennings found that while individuals descended from a single paramecium vary in size (fig. 89) , yet the population from a large individual is the same as the population derived from a small individual. In other words, selection produces no result and the probable explanation is, of course, that the different sizes of individuals are due to the environment, while the constancy of the type is genetic. Jennings found a number of races of paramecium of different sizes living under natural conditions. ”

“ The results indicate a highly shared genetic origin of monogenic, complex and environmental diseases. Moreover, most diseases cannot be attributed to a single gene but to defects of several genes. Interestingly, these genes are likely to participate in a core set of biological processes. ”

“ Patterns of inheritance are complicated by the fact that one set of genes for all traits are inherited from each parent, so that any given individual will normally have two complementary sets. This overall genetic context is also very important; many traits may only be seen where genes for alternative forms are absent. ”

“ And he said that we are all, regardless of race, genetically 99.9 percent the same. ”

“ I am simply pointing out that at the rate at which we are going the whole genetic engineering technology will end up in the hands of the political system to be used for the complete control and subjugation of man. ”

“ In man and in domesticated animals we find that individuals appear occasionally that are defective in one or another respect. Some of the defects are inherited. Rarely a new one appears that has not been seen before. But the majority of them are reappearances of characters that have been carried under the surface as recessive genes in the germ-plasm. Today we recognize that each of these modifications, if recessive, has first arisen as a mutational change in a single gene before it appeared on the surface as a character by the coming together of two such genes. ”

“ This so-called purity of the germ-cells, then, may be expressed in this way: "The hybrid, whatever its own character, produces ripe germ-cells, which produce only the pure character of one parent or of the other" (Castle) .
Although Mendel's discovery was for a long time overlooked, happily the facts were re-discovered, and at the present time extensive experiments are being made with animals to test this law: experiments in the inheritance of poultry, the inheritance of fur in guinea-pigs, of erectness in the ears of rabbits, etc., etc. ”

“ 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. ”

“ In this genotype theory we are dealing only with a new and more refined aspect of the “theory of pure gametes.” It is not a necessary part of mendelism, not even an original part; but it is very important for us to know whether it is true or not. For if it is true, selection unattended by hybridization is largely a waste of time, as De Vries and Johannsen have maintained, and Jennings and Pearl have reiterated. ”

“ An individual may be pure bred in certain of its characteristics, hybrid in others. Practically there is no such thing as an individual which is either pure bred or hybrid in all its traits. One of the chief contributions, then, of Mendelism to the subjects of Heredity and Eugenics is this—that a pure bred may be derived from a hybrid in one generation: the pure bred produced by a long series of hybrid individuals is just as pure as the pure bred which has never had a hybrid in its ancestry. ”

“ 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. ”

“ On the contrary the fifty cross-breds, as stated above, have mixed offspring. But these offspring, again, in their numerical proportions, follow the same law, namely, that there are three dominants to one recessive. The recessives are pure like those of the last generation, but the dominants can, by further self-fertilisation, and examination or cultivation of the seeds produced, be again shown to be made up of pure dominants and cross-breds in the same proportion of one dominant to two cross-breds. ”

“ 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. ”

“ 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. ”

“ For the brain of a child is not descended from the brain of his parent, but from a germ-cell carried by that parent.
Persistence of Mendelian Factors Argues Against Such a Mode of Inheritance.—On the face of things, the apparent inviolability of Mendelian factors which may remain unexpressed in the germ for one or many generations—indeed the whole matter of genotypical differences in the gametes of the same individual—shows the improbability of somatic interference with the germ-plasm. ”

“ 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. ”

“ Such a result is produced by random assortment of the chromosomes in gamete and zygote formation, and occurs when the genes for two or more pairs of contrasted characters are located in different chromosome pairs. The phenomenon is formulated in Mendel’s Second Law, the law of independent assortment. The novelty, however, of the true-breeding forms thus produced is not absolute, but relative. There is no origination of new hereditary factors. It is simply a recombination of the old genes of different stocks, the genes themselves undergoing no intrinsic alteration. ”

“ But variability, in the sense of division of gonads into heterogeneous gametes, may surely be due to causes other than crossing. This we cannot doubt. Cross-fertilization of the zygote producing those gametes is one of the causes of such heterogeneity among them. We cannot suppose it to be the sole cause of this phenomenon.
When Mendel asserts the purity of the germ-cells of cross-breds he cannot be understood to mean that they are more pure than those of the original parental races. ”

“ These go over into the first generation hybrids, and are sorted out in the germ cells of the hybrid. Consequently, when the F1’s are inbred, there are all sorts of recombinations of the minor factors. This explains the greater variability of the second generation.
It is probable that in most of our domesticated animals, including man, much of the variability is due to multiple factors, which makes a study of inheritance in these groups extremely difficult, especially when, as in the case of man, the number of offspring from a pair is small, and critical combinations for study can not be made. ”

“ It is obvious that the nucleoplasm of each antecedent generation must be represented in any germ-nucleus in an amount which becomes less as the number of intervening generations becomes greater; and the proportion can be calculated after the manner in which breeders, when crossing races, determine the proportion of pure blood which is contained in any of the descendants. Thus while the germ-plasm of the father or mother constitutes half the nucleus of any fertilized ovum, that of a grandparent only forms a quarter, and that of the tenth generation backwards only 1/1024, and so on. ”

“ Or in Mendelian phraseology, the defective traits are recessive and are dominated by the normality of the other parent. Such offspring, however, while apparently normal of body are not normal of germ-plasm, inasmuch as half of their germ-cells will carry the abnormality of the defective parent as earlier explained (page 119) under Mendelism. We have already seen (page 119) how by continually marrying into strong strains the liability to recessive defect can be diluted out until the descendants are no more likely to have defective children than are members of our ordinary population. ”