Recessive allele

“ In carrying on a stock of a lethal, advantage can be taken of linkage. A lethal factor has a definite locus in the chromosome; if, then, a lethal-bearing female is crossed to a male of another stock with a recessive character whose factor lies in the X chromosome very close to the lethal factor, half the daughters will have lethal in one X and the recessive in the other. ”

“ Thus black guinea-pigs of formula BB are duplex with regard to the determiner for black color, individuals of formula Bb are simplex with reference to this determiner, and those of formula bb are nulliplex.
A heterozygote in which dominance prevails can be identified with certainty by breeding to a known recessive and noting the kind of offspring produced. If the individual was really a heterozygote, approximately fifty per cent. of the offspring should be of the recessive type. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ For example, those with most black pigment and those with least black taken from a population continuously varying in this respect, were separately bred; but almost always the selection led to no sensible change in the position of the mean of the [Pg 221] population. The variations in these cases were evidently fluctuational. In some instances, however, real genetic differences were met with, and strains exhibiting them were, as usual, rapidly fixed.
Tower points out that several of the varieties (or species, as he prefers to call them) were obviously recessive to decemlineata. ”

“ If it should prove to be true that varieties when crossed do not, in the course of eventual inter-breeding, produce intermediate forms as hybrids, but that characters are either dominant or recessive, and that breeds result having pure unmixed characters—we should, in proportion as the Mendelian law is shown to apply to all tissues and organs and to a majority of organisms, have before us a very important and determining principle in all that relates to heredity and variation. ”

“ 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. ”

“ In the male there are two classes of sperm, one carrying the X chromosome and the other carrying the Y chromosome. Any egg fertilized by an X sperm produces a female; any egg fertilized by a Y sperm produces a male. The scheme of inheritance is as follows.
The sons get their single X chromosome from their mother, and should therefore show any character whose gen is carried by such a chromosome. In sex-linked inheritance all sons show the characters of their mother. A male transmits his sex-linked character to his daughters, who show it if dominant and conceal it if recessive. ”

“ 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. ”

“ If a heterozygous bird be mated to a recessive the variability of the offspring is much increased, owing to the occurrence in the progeny of both DR and RR individuals (table 40) . The offspring do not, to be sure, fall into two distinct and well-defined types, as in typical Mendelian cases; but one part of the range of variation agrees fairly with that of pure RR's, i. e., Brahmas, and the remainder with that of heterozygotes. ”

“ If they were proved to enter from without, like pathogenic organisms, we should have to account for the extraordinary fact that they are distributed with fair constancy to half the gametes of the heterozygote.
In proportion as the nature of dominants grows more clear so does it become increasingly difficult to make any plausible suggestion as to their possible derivation. On the other hand the origin of a recessive variety by the loss of a factor is a process so readily imagined that our wonder is rather that the phenomenon is not observed far more often. ”

“ He finds that in the process of hybridization there are certain characteristics which are transmitted entire to the offspring, and are termed dominant, others which seem to disappear or become latent in the process, which he terms recessive. ”

“ Recognition of the distinction between dominant and recessive characters has, it must be conceded, created a very serious obstacle in the way of any rational and concrete theory of evolution. While variations of all kinds could be regarded as manifestations of some mysterious instability of organisms this difficulty did not occur to the mind of evolutionists. To most of those who have taken part in genetic analysis it has become a permanent and continual obsession. ”

“ Accordingly, any pair of recessive individuals may form the beginning of a pure race of recessives, but in starting a pure race of dominants we must test each animal used, to make sure that it does not contain, unseen, the recessive character. Otherwise we may keep getting mixed lots of offspring. The simplest and surest way of making such a test is to mate the dominant animal with a recessive. For in that case, if the dominant is pure, all the offspring will be dominants; but if the dominant parent is a hybrid, half the offspring will be recessives, half dominants. ”

“ 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 ”

“ 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. ”

“ In a population in equilibrium consisting of homozygous dominants, heterozygous dominants and recessives the last named class comprises 2.8% of the total: assuming that a 10% selection rate now operates in its favour as opposed to the two classes of dominants—in how many generations will the recessive come to constitute one-quarter of the population? ”

“ Owing to the extreme imperfection of dominance it is rarely possible to say with certainty from inspection whether a given bird has germ-cells dominant or recessive, or 52 both, with reference to booting; only breeding enables us to make a decision. There is an exception, however, in the case of pure extracted recessives. They are distinguished by heavy booting and produce only booted offspring. I propose to give, in detail, the matings of these later generations and their progeny, the families being arranged in decreasing order of average grade of booting (table 38) . ”

“ When we see the phenomenon in this light we realise the obscurity of the problem. The appearance of recessive varieties is comparatively easy to understand. All that is implied is the omission of a constituent. How precisely the omission is effected we cannot suggest, but it is not very difficult to suppose that by some mechanical fault of cell-division a power may be lost. Such variation by unpacking, or analysis of a previously existing complex, though unaccountable, is not inconceivable. But whence come the new dominants? ”

“ Thus by continually marrying into strong strains liability to manifest any recessive defect can be diminished in a few generations until the descendants are no more likely to have defective children than are members of our ordinary population.
The proportion in which the recessive defect would appear in successive generations if all persons in a given generation married only normal individuals who were non-carriers is indicated in the following table where AA indicates a normal individual, Aa one who is normal but a carrier, and aa an individual with the imperfection expressed ”

“ Mutations at strongly constrained synonymous sites should never rise above low frequency in the population and certainly will never fix, barring tight linkage to a very advantageous allele or a shift in the functional properties of the site. ”

“ For all practical purposes in plant breeding the mere fact of segregation is of greatest importance and the complexity of recent Mendelian interpretations need not bother us. Suffice it to say that most plant breeders have accepted the explanation that the recessive character is simply the lack or absence of the character in question, while the dominant character is its presence. ”

“ 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. ”

“ This failure even of the extracted dominants completely to stop the development of the tail gives a measure of the weakness of the inhibitor in this case. Also, in table 28, matings are varied. Some are probably matings of two heterozygotes, others of two recessives, and others still of a recessive with a heterozygote. On our hypothesis we should expect some of the families of the mated hybrids to show evidence of the inhibiting factor and others to show no such evidence. In those families in which small tail appears it is found in about 19 per cent of the cases. ”

“ 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 evident from these data that there must be present in the sex-chromosome a gen that causes the death of every male that receives this chromosome, and that this lethal factor lies very close to the factor for white eyes. ”

“ The result of this is that the relative number of dominants and recessives increases in successive generations, whereas the relative number of mixed dominant-recessives decreases, and in a few generations a hybrid race will revert in large part to its parental types if continued hybridization is prevented. ”

“ 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. ”

“ If dominance is supposed to be a criterion as to "presence," then it should be pointed out that among the mutants of Drosophila a number of dominant types occur. But clearly we are not justified by these criteria in inferring anything whatever in regard to the nature of the change that takes place in the germ-plasm. Probably the only data which give a basis for attempting to decide the nature of the change in the germ-plasm are from cases where multiple allelomorphs are found. ”