Recessiveness

“ As offspring tend to resemble grandparents almost as much as parents, and as a line of close-bred ancestry is generally prepotent, so newly-originated varieties have always a tendency to reversion. This is pretty sure to show itself in some of the progeny of the earlier generations, and the breeder has to guard against it by rigid selection. But the older the variety is—that is, the longer the series of generations in which it has come true from seed—the less the chance of reversion: for now, to be like the immediate parents, is also to be like a long line of ancestry ”

“ There is a class of cases of inheritance, due to the XY chromosomes, that is called sex linked inheritance. It is shown both by mutant characters and characters of wild species.
For instance, white eye color in Drosophila shows sex linked inheritance. If a white eyed male is mated to a wild red eyed female (fig. 35) all the offspring have red eyes. If these are inbred, there are three red to one white eyed offspring, but white eyes occur only in the males. ”

“ It has become gradually clear that while the fertilised egg-cell gives rise to body-cells which become specialised, it also gives rise to unspecialised descendant-cells, which take no share in body-making, but become the germ-cells—the potential starting-points of another generation. A second great step was the accumulation of facts of inheritance showing that all sorts of qualities innate or inborn in the parents, essential and trivial, normal and abnormal, bodily and mental, may be transmitted to the offspring as part of the organic heritage. ”

“ A pure recessive individual is one arising from the union of two gametes both carrying recessives. If a heterozygote is bred with a pure recessive the offspring are half heterozygote and half recessive. The heterozygote individual in typical cases shows the dominant character. In the formation of its gametes when the reduction division of the chromosomes takes place, half of them receive the dominant character, half the recessive. When the division in the gametes of the recessive individual takes place its gametes all contain the recessive character. ”

“ The recessive passes from 2.8% to 11.1% of the population in 36 generations, and from 11.1% to 25% in a further 16 generations—i.e. under a 10% selection rate in its favour the proportion of the recessive rises from 2.8% to 25% in 52 generations. ”

“ For in all higher animals, at least, offspring may possess some of the characteristics originally present in either of two lines of ancestry, and this commingling of such possessions is possible only through sexual reproduction. As has already been seen, in the pairing of chromosomes previous to reduction, the corresponding members of a pair always come together so that in the final segregation each gamete is sure to have one of each kind although whether a given chromosome of the haploid set is of maternal or paternal origin seems to be merely a matter of chance. ”

“ 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. ”

“ Indeed the evidence which proves Heredity in its smaller manifestations is the same evidence which proves Variation; since it is only when there occur variations that the inheritance of anything beyond the structural peculiarities of the species can be proved. It remains here, however, to be observed that the transmission of variations is itself variable; and that it varies both in the direction of decrease and in the direction of increase. An individual trait of one parent may be so counteracted by the influence of the other parent, that it may not appear in the offspring ”

“ Of the second generation, one-half the individuals are still mixed, bearing the dominant characteristic externally and the other hidden; one-fourth are pure dominants and one-fourth are pure recessives. In future generations the mixed or hybrid individuals again give birth to mixed and pure types apportioned as before, thus continuing until all offspring become ultimately pure. ”

“ From the standpoint of heredity, however, the important consideration is to determine if these experimentally induced changes have been reflected on to the germ-cells so that they reappear in the offspring of the modified individuals.
It has been found that in some cases where male and female are of different color, the color of the female can be changed to that of the male by altering the conditions of temperature. In certain cases types can be changed by cold so that they resemble varieties of the same species found farther north, and by heat, varieties found farther south. ”

“ This condition in mammals introduces a complicating factor which is likely to obscure the whole issue unless we bear it constantly in mind. In other words, we must discriminate sharply, in the discussion of inheritance in man, for instance, between two classes of influences which may exist in the infant at birth, that is, which are congenital; namely, those which were truly inherent—were in the germ-cells—at the very inception of the young individual, and (2) those which might later have been derived from either parent by the yet unborn offspring. ”

“ Suppose then the offspring of any parent to vary in many directions. By cross-breeding among themselves and with the parent stock, these are usually merged in a common type, their differences pooled, and the species remains fixed or else advances slowly by natural selection, along one line, as physical conditions change in geological time. But from time to time there arises a variation in the reproductive organs of some individuals, of such kind that these individuals are fertile among themselves, but sterile or less fertile with other varieties and with the parent stock. ”

“ Most of the genes in any given population, good or bad, are so hidden that it is practically impossible to predict what the offspring of any particular couple will be.
The recessive genes have vastly increased through the operation of human “melting pots” all over the world in the last few generations. One result is that minority races tend to become absorbed in majorities. ”

“ Nor are we likely to forget, in the midst of our commemoration of Darwin's birth and of "The Origin of Species," that the whole doctrine of organic evolution rests upon the facts of heredity. Whatever the factors that determine racial descent—fluctuating variation, or the sudden change of type, use and disuse, natural, artificial, sexual or organic selection—both continuity in the process of bionomic change and maintenance of the change once produced, demand the conception of a hereditary likeness. ”

“ 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. ”

“ That existing forms of life have been evolved from other forms, and that these ancestral forms have been different from those derived from them, is shown by the occasional appearance of antecedent and abandoned types of structure among the offspring of existing species. Occasionally a human child is born strangely unlike its parents, but bearing an unmistakable resemblance in looks and disposition to his great-grandfather or some other remote ancestor. This is atavism, that tendency to revert to ancestral types which is prevalent among all animals. ”

“ It has been found by experiment that when two cross-bred (or hybrid) dominant animals are mated together, the offspring consist of a mixture of dominant with recessive individuals in approximately the proportions, three dominant to one recessive. Further, when a hybrid dominant is mated with a recessive animal, half the offspring are hybrid dominants, half recessives. ”

“ The nature of the traits can be told only by a study of the ancestry. Of course, characters may be latent or recessive, but this is also the case in the population at large, and the chance of unpleasant results is so small, when no instance can be found in the ancestry, that it can be disregarded. If the same congenital defect or undesirable trait does not appear in the three previous generations of two cousins, including collaterals, the individuals need not be discouraged from marrying if they want to. ”

“ In pre-Mendelian times all that was known was that some forms could freely interbreed without diminution of fertility in the product, while others could not. But now we find that, by virtue of segregation, from one and the same pair of parents, or even, in the case of hermaphrodites, from one and the same individual, offspring commonly arises showing among themselves exactly such differences as distinguish species—and very good species too. ”

“ The critical test is afforded by the F2 generation (tables 48 and 49) . By hypothesis, 25 per cent of the offspring are expected to be pure ("extracted") recessives, and the same number pure dominants; and also, by hypothesis, the recessives are massed at or near one grade while the dominants are variable. Now, as a matter of fact, the upper 25 per cent range over 5 to 7 grades, while the lower 25 per cent are nearly massed in grade 1 65 (21 per cent are so massed in one table, 17 per cent in the other) . ”

“ Offspring Derived from One Parent Only.—So far in our reckoning of heredity we have counted elements from both father and mother, and the complications which arise from such a double ancestry are manifestly very perplexing ones. If we could do away with the elements of sex and find offspring that are derived from one parent only, it would seemingly simplify our problem very much for we should thus have a direct line of descent, free from intermingling. ”

“ The progeny of the recessives remained pure recessive; and in any number of subsequent generations never produced the dominant type again, that is, never reverted to the original parent, whose qualities had failed to appear in the second generation. When the seeds obtained by self-fertilizing the plants with the dominant characteristics were sown, it was found by the test of progeny that the dominants were not all of like nature, but consisted of two classes--first, some which gave rise to pure dominants ”

“ We know that the germ-plasm of each individual is not a simple substance, but possesses a very complex composition, for it consists of a number of ancestral germ-plasms represented in very different proportions. Now, although we cannot learn anything directly about the processes of growth of the germ-plasm, and its resulting ontogenetic stages, yet we do know, chiefly from observations upon man, that the characters of ancestors appear in the offspring in very different combinations and in very different degrees of strength. ”

“ The outstanding fact that emerges prominently from the study of the local forms is that when two definite types, nearly allied, and capable of interbreeding with production of fertile offspring, meet together in the region where their distributions overlap, though intergrades are habitually found, there is no normally or uniformly intermediate population occupying the area of intergradation. Such phenomena as these must, I think, be admitted to have great weight in any attempt to construct a theory of evolution. ”

“ 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. ”

“ In such a case, the germ-plasm of the first generation of a species which enters upon sexual reproduction must still be entirely homogeneous; the hereditary substance must, in each individual, consist of many minute units, each of which is exactly like the other, and each of which contains within itself the tendency to transmit, under certain circumstances, the whole of the characters of the parent to a new organism—the offspring. ”

“ 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. ”