Genotype

“ The person nourishes and protects the germ, and in this sense the person is merely the carrier of the germ plasm, the mortal trustee of an immortal substance.
This contrast of the germ and the person, of the undeveloped and the developed organism, is fundamental in all modern work on heredity. It was especially emphasized by Weismann in his germ plasm theory and recently it has been given prominence by Johannsen under the terms genotype and phenotype; the genotype is the fundamental hereditary constitution of an organism—it is the germinal type ”

“ Figures Model of Two Genotypes with Forward MutationEach genotype Ai has its own fitness wi and mutational loss μi. Mutation is asymmetric, so that A1 gives rise to A2, but not vice versa.Dynamic Approach to the Quasispecies Equilibrium, for the Genotypes Illustrated in Figure 1Drawn for w1 = 1.5, w2 = 1.0, μ1 = 0.7, and μ2 = 0.6. ”

“ If their habits varied considerably it would not necessarily mean that their relationship was more distant, but similarities can usually be considered indicative of affinities because they are the phenotypic expression of the partially unaltered genotype of the common ancestor. ”

“ In addition, researchers have increasingly realized that there are many levels of controls along the paths from genotypes to phenotypes, resulting in a weaker relationship between genotypes and phenotypes [1] that may or may not be captured using traditional linkage or association approaches. Furthermore, linkage analysis usually can only identify chromosomal intervals that may contain up to hundreds of candidate genes owning to the limited number of crossovers in sampled families. Genome-wide association studies may also return many regions that show moderate to high signals. ”

“ In such an observation two things are strikingly exemplified, (1) the fact of the permanence of the unit, and (2) the fact that a mixture of types in the family means that one or other parent is cross-bred in some respect, and is giving off gametes of more than one type.
The problem of heredity is thus a problem primarily analytical. We have to detect and enumerate the factors out of which the bodies of animals and plants are built up, and the laws of their distribution among the germ-cells. ”

“ 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. ”

“ We may, then, allow for all individual variation while defining species as populations with common heredity. The older definition of a species as a group of similar (implying nearly identical) individuals fails because of the amount of variability actually found in nature. Definitions of species as groups of individuals distinguished by a definite number of diagnostic characters, or by certain degrees of difference from other species lead to artificial concepts that take no adequate account of individual variation, Mendelian inheritance, hybridization, or mutation. ”

“ The distinction between these is that where the germ cells are different, the difference will be transmitted by heredity to future generations; if it is a modification that comes on in connection with development and is not due to a difference in the germ cell, it will not show itself in future generations. What the animal and plant breeders have to do is to watch for changes in their stock, and when the kinds that they are looking for appear, they breed them in the hope that the desired features will prove to be hereditary, so that a race can be established showing them. ”

“ 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 a single period, in a limited area, there must have existed a population from which all of the present-day species, with their varying grades of relationships, have developed. This history of the expansion of a single genetic stock into 93 distinct populations, by processes of mutation, isolation, and on occasion subsequent recombination into hybrid populations, is the story we have been unfolding in this study, and which we are now ready to fit into the geologic time and the geographic areas in which speciation probably proceeded in the genus. ”

“ But when we proceed to ask how the germ-cells will be constituted in the case of an individual who is cross-bred in some respect, containing that is to say, an ingredient from the one side of his parentage and not from the other, the answer is entirely contrary to all the preconceptions which either science or common sense had formed about heredity. ”

“ 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. ”

“ 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. ”

“ But do such inter-specific hybrids ever give rise to populations that deserve to be called species?
It must be remembered that transition zone populations grade in every direction into the pure populations between which they are hybrid. Any portion of such a hybrid population is different biometrically from any other portion of that population. The genes available at one point in the transition zone are not equally available at every other point in the zone. There is no common heredity within the population. ”

“ But if such a population, of hybrid origin, should in some way become isolated, then it might in the course of time become a fairly uniform population. Relieved from the continual introduction of genes from the parental stocks, the hybrids might finally achieve a thoro exchange of genes thruout the whole of the population. Then a sample taken from any part of the range would vary within the same wide but uniform limits typical of every other sample of the population. It would then satisfy our concept of a species, for it would have a common heredity. ”

“ One half of the egg cells, likewise, contain the dominant, and one half the recessive character.
Now, if we could pick out at random any one hundred pollen or male cells to fertilize any one hundred egg or female cells, we can see that there are equal chances for four results. A dominant male cell might meet a dominant female cell, a dominant male cell a recessive female cell, a recessive male cell a dominant female cell, and a recessive male cell a recessive female cell. ”

“ 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. ”

“ ESSENTIAL PREMISES The mechanism of heredity lies in a single germ cell carrying the character-determining chromosomes which consist of organized chains of genes. In the character and organization of these genes in the fertilized cell we find the complete "blueprint" for the future individual in so far as it is to be determined by heredity. In the twenty-four pairs of chromosomes in the fertilized human germ cell we find the long and diversified heritage of each parent represented through the union of the sperm and the ovum. ”

“ The inclusion of a genealogy requires that it be found in a paper that specifically addresses one of the supergroups or analyzes broad eukaryotic diversity. The genealogies must also include adequate sampling—two-member taxa per supergroup—from at least two of the six supergroups to allow for the comparison of supergroup monophyly. In cases where multiple gene trees are presented we display the authors' findings as multiple entries when the trees are not congruent or as a single entry when the trees are concordant. ”

“ To make clear the application of the terms chemical and mechanical to the problem of Genetics the nature of that problem must be more fully described. In its most concrete form this problem is expressed in the question, how does a cell divide? If the organism is unicellular, and the single cell is the whole body, then the process of heredity is accomplished in the single operation of cell-division. Similarly in animals and plants whose bodies are made up of many cells, the whole process of heredity is accomplished in the cell-divisions by which the germ-cells are formed. ”

“ And lastly, the problem of hereditary transmission has been rendered more definite and approachable by the discovery that the male germ is likewise a single cell, that fertilization of the egg is therefore the union of two cells, and that in consequence the mechanism of inheritance must be stowed away somehow in these two cells.
Heredity in Unicellular Forms.—In unicellular animals one can readily see how it is possible for an [Pg 23] individual always to give rise to its own kind. ”

“ According to their view, in the formation of the germ cells a segregation of the unit pairs occurs—that is to say, the peculiar body or ferment is handed on to one daughter-cell but not to the other. A similar kind of segregation may take place in the formation of the repeated parts of an organism, so that symmetrical repetition may be compared with normal heredity, and be due to the presence of similar factors in the divisions of the embryonic cells, whilst the differentiation of repeated parts may be due to the unequal distribution of such factors and be comparable with variation. ”

“ 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. ”

“ A question that is being vigorously debated at this time concerns the possible effects of selection on the hereditary factors. Are the genes fixed both qualitatively and quantitatively or does a given gene vary in potency under different conditions and in different individuals? In the former case selection can only separate the existing genes into separate pure strains. But if the gene be quantitatively variable, then selection will result in the establishment of new types. ”

“ We have now established the bases for a practicable eugenics program. Men differ; these differences are inherited; therefore the make-up of the race can be changed by any method which will alter the relative proportions of the contributions which different classes of men make to the following generation.
For applied eugenics, it is sufficient to know that mental and physical differences are inherited; the exact manner of inheritance it would be important to know, but even without a knowledge of the details of the mechanism of heredity, a program of eugenics is yet wholly feasible. ”

“ In conclusion, summing up all our arguments, we may state that there is a broad analogy between breeding selection in the widest sense of the word, including variety testing, race improvement and the trial of the breeding ability on one side, and natural selection on the other. This analogy however, points to the importance of the selection between elementary species, and the very subordinate role of intraspecific selection in nature. It strongly supports our view of the origin of species by mutation instead of continuous selection. ”

“ For it is obvious that the contributions of the male and female gametes may in respect of any of the ingredients be either the same, or different. In any case in which the contribution made by the two cells is the same, the resulting organism—in our example the man—is, as we call it, pure-bred for that ingredient, and in all respects in which the contribution from the two sides of the parentage is dissimilar the resulting organism is cross-bred. ”

“ 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. ”