Biogenetic law

“ It is only when one appreciates the cenogenetic processes in relation to the palingenetic, and when one takes careful account of the changes which the latter may suffer from the former, that the radical importance of the biogenetic law is recognised, and it is felt to be the most illuminating principle in the science of evolution. ”

“ It is recognized, of course, that comparative embryology can furnish embryological homologies analogous to the homologies of comparative anatomy, but it is now generally acknowledged that the view, which regards the embryological process as an abridged repetition of the various states through which the species has passed in its evolutionary career must be definitively abandoned, and that, as a general law of organic development, the biogenetic principle has been thoroughly discredited. ”

“ These factors are not modified in the least by the environment, and their peculiarities are faithfully transmitted in heredity with all the precision of chemical law. But even these factors are all within the bounds of the species. There is not a shred of scientific evidence that either natural or artificial devices have originated a single genetic factor that was not all the time potentially latent in the ancestry, capable of being produced at will by the proper combination.
It is a universal law of living things that all forms left to themselves tend to degenerate. ”

“ Certain limitations to the law are met with because the relatively small number of chromosomes involves linkage of genes, because of the occasional interchange of groups of genes between homologous chromosomes, and because the relative activity or potency of any particular gene may differ in different races, and, finally, because the normal activity of any given gene may be modified or inhibited by the action of other genes. ”

“ When such lines and forms of organic development prevail, the individual, as the cell of the body, becomes soon senescent, drifting inevitably into age, decay, and death. The great biologist and embryologist, Professor Minot, describes this downward course of organic evolution, as the Law of Genetic Restriction: "The development runs in one direction, and ends in the production of structure, which, if it is pursued to its legitimate terminus, results in degeneration and death." Societies, developing on lines of organic growth, follow the Law of Genetic Restriction. ”

“ 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. ”

“ The most important result of this law is that it leads to uniformity of character in the individuals of the same species. In the case of certain hermaphrodites, which probably intercross only at long intervals of time, and with unisexual animals inhabiting somewhat separated localities, which can only occasionally come into contact and pair, the greater vigour and fertility of the crossed offspring will ultimately tend to give uniformity of character. But when we go beyond the limits of the same species, free intercrossing is barred by the law of sterility. ”

“ If two people, each serving as a carrier for such a gene, have children, a sperm cell carrying a lethal may fertilize an egg cell carrying the same type of lethal, with sad results.
Every species, including man, includes individuals who carry undesirable genes. These undesirable genes may be passed along for generations, even if dominant, before natural selection culls them out. The more seriously undesirable they are, the more quickly they are removed, but even outright lethal genes will be included among the chromosomes from generation to generation provided they are recessive. ”

“ If so, then perhaps genome assembly methods are the best. What if instead, as in the sharpshooter example, we are trying to have each bin include every fragment that came from a particular species, knowledge which may be useful for predicting community metabolic potential? If the level of genetic polymorphism among individual cells from the same species is high, then genome assembly methods may not work well (the polymorphisms will break up assemblies) . A better approach might be to look for species-specific “word” frequencies in the DNA, such as ones created by patterns in codon usage. ”

“ Their incompatibility may well be of the same nature as that of the classes in Cardamine pratensis.
Returning now to the problem of inter-specific sterility; we note, as I have said, the absence of contemporary evidence that variation can confer on a variety the power to form a sterile hybrid with the parent species. The considerations based on this want of evidence have for a long while been familiar to all who have discussed evolutionary theories, and it is worth observing the exact reason why the difficulty strikes us now with a new and special force. ”

“ Artificial insemination, for example, permitted the propagation of valuable genomes without the sire being physically present, thereby allowing superior genetics to be spread beyond relatively small geographical areas. ”

“ 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. ”

“ 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. ”

“ There is a growing impression that a good deal of feeblemindedness and insanity are environmental rather than hereditary traits; poverty, malnutrition, and especially syphilis are said to play a considerable rôle in their production. It is unsafe therefore to conclude that the human germ-plasm is as badly contaminated as some pessimists seem to think.
If we turn now more directly to special kinds of human inheritance we shall find a great deal of evidence showing that the same laws of inheritance that hold for animals and for plants apply to man. ”

“ While severe mutant alleles that eliminate protein function represent the null state and confer lethality, hypomorphic alleles simply reduce protein function below an optimal level, resulting in a phenotype recognized as disease. In this manner, lethal genes can have mutations that only reduce protein function and therefore present as disease in the human population, allowing for inheritance of these more mild alleles in genetic diseases. ”

“ Broad claims are made about both the potential benefits and the potential hazards of genetically engineered organisms. Use of recombinant DNA may lead to welcome advances in such areas as food production and disease control. ”

“ 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. ”

“ From a breeder’s perspective, phenotypes resulting from sexual reproduction cannot be predicted—that is, the characteristics of the offspring from a mating may be estimated, but not predicted with certainty. Nuclear transfer, the most advanced of these technologies, does not require fertilization and allows for the propagation of known genotypes and phenotypes without the risk of genetic reshuffling. ”

“ But it may happen that, at the reduction of ids during the development of the germ-cells, an id-combination [Pg 50] with wholly or almost wholly unchanged ids may come together in one germ-cell, and if another sperm-cell of this kind meets with an egg-cell similarly constituted, an individual of the old species must arise. But this must—on our assumption—be at a disadvantage as compared with the transformed individuals in the struggle for existence, and will perish in it, and therefore the number of unmodified ids in the germ-plasm of the species will gradually diminish. ”

“ Human Social Genomics
Initial indications that social environments might significantly affect the functional activity of the human genome came from studies dissecting leukocyte gene expression profiles into genetic and environmental components [8] , [9] . Gibson and colleagues found that ∼5% of genes expressed in leukocytes showed appreciable genetic regulation (e.g., via expression quantitative trait loci) , whereas >50% showed significant differences in expression across pastoral, rural, and urban social environments [9] . ”

“ Linking polymorphism with divergence data, we further find that the fraction of synonymous sites exposed to strong purifying selection is higher for those positions that show slower evolution on the Drosophila phylogeny. The function underlying the inferred strong constraint appears to be separate from splicing enhancers, nucleosome positioning, and the translational optimization generating canonical codon bias. The fraction of synonymous sites under strong constraint within a gene correlates well with gene expression, particularly in the mid-late embryo, pupae, and adult developmental stages. ”

“ GENOME EDITING
For decades, people have been yearning for being able to permanently alter the DNA, by inserting or deleting genes or specific pieces of DNA. The reason is that this process can be applied to agronomy, veterinary and human medicine. ”

“ Even so, strong selection, rather than a mutational difference, would still underlie our signal, as genetic draft cannot alter the frequency of synonymous mutations that are simply absent from the population. On the other hand, sweeps of weakly advantageous alleles in coding sequences could eliminate polymorphism in 4D sites more so than in short introns. Narrow selective sweeps in coding sequences reducing variation at otherwise neutral 4D sites is, however, an unlikely explanation for our observations. ”

“ D. The Implications of Epigenetic Reprogramming for Clones and their Progeny (Chapter IV) Epigenetics has been defined as the study of stable alterations in gene expression potentials that arise during development and cell proliferation. In sexual reproduction, a new diploid genome is created by the fusion of two haploid genomes. The subsequent expression of that genome into a functional organism is governed by a "program." There are several examples of epigenetic control of gene expression, of which DNA methylation is likely the best characterized. ”

“ 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 identified 1,005 genes, either as isolated genes or in clusters positionally biased toward rearrangement-prone genomic regions, that produced relative hybridization signals unique to one or more of the hominoid lineages. Measured as a function of the evolutionary age of each lineage, genes showing copy number expansions were most pronounced in human (134) and include a number of genes thought to be involved in the structure and function of the brain. This work represents, to our knowledge, the first genome-wide gene-based survey of gene duplication across hominoid species. ”

“ If synonymous sites harbor highly deleterious variants under strong purifying selection, then that must change our view of the functional importance of synonymous sites and their potential role in genetic disease, as a possible source for adaptation, and as the neutral foil in tests for selection. ”

“ While mutational cold spots only lead to a lower number of SNPs, under strong purifying selection some mutations should still be observable at very low frequencies in a deep enough sample of the population. Weak selection, meanwhile, will affect the shape of the spectrum beyond the rare alleles and can be estimated from that. Combined, the lack of polymorphism and the excess of rare variants from a genome-wide, deep sample, could give the necessary power to quantify the intensity of the strong constraint and the fraction of sites it affects. ”