Chapter 33 Chapter 18 Various theories about genes-1

The laws of Mendelian genetics provide an excellent account of the phenomenon of discrete variation.These laws are easy to apply whenever definite traits are involved, such as green versus yellow in peas, smooth versus wrinkled. Hundreds of thousands of articles were published after 1900 demonstrating the existence of Mendelian heredity in a variety of animals and plants, thus endowing Mendelian genetics with any observable discontinuous variation. Yet the objection that Mendelian inheritance was not universally applicable was popular for a considerable time.It would be a mistake to attribute this objection entirely to ignorance or conservatism, since such an explanation would be too simplistic.In fact the opponents felt that they had every reason to object.In addition, to be fair, they do not deny some Mendelian inheritance phenomena, what they object to is only attributing all hereditary phenomena to Mendelian inheritance.Since many of these opponents are biologists of the first rank, it is necessary to analyze the reasons they serve.

Recent historians tend to forget that at the turn of the 19th and 20th centuries most Darwinist zoologists and botanists were concerned with the phenomenon of heredity because it had to do with the problem of species and with the theory of evolution.These Darwinists therefore read only the two Mendelists who were most interested in the question of evolution, de Vry and Bateson, and their views prompted these Darwinists to stand firmly against it.Both de Vry and Bateson preached the discontinuity of genetic phenomena to demonstrate the discontinuity of evolutionary origins.Both were essentialists and catastrophists (see Chapter 12), and neither believed much in natural selection.Their point of view is thus quite different from that of the Darwinists, who see evidence of gradual evolution everywhere in nature.Since the Mendelians claim that the mode of hereditary variation (that is, discontinuity) is closely related to the mode of evolution, and since they themselves believe that evolution is not gradual and continuous; the Darwinian naturalist is compelled to propose some non-Mendelian Del-like, continuous inheritance to explain gradual evolution (Mayr and Provine, 1980).

As far as naturalists are concerned, the greatest weakness of Mendelianism is that it does not account for continuous variation.At that time, almost everyone still recognized the duality of variation (continuous variation and discontinuous variation), and Mendelianism was considered to have no explanation for quantitative variation (quantitstivevariation).We recall that Weissmann, de Vry, and others in the 1880s and 1890s explained quantitative inheritance in terms of the difference in the number of (identical) protons or bioforms provided by the two parents.De Vry once said, "The relative number of pans may change, some may increase, others may decrease or disappear almost entirely... Finally, clusters of individual pans may also vary. All these processes are sufficient to explain violent fluctuations ( individual, continuous) variation” (1900: 74).This interpretation fell through when Mendelianism (which provided only one factor per parent for relative traits) was accepted.Continuous variation is not accounted for in this case.Nor have I found any alternative explanations for this doctrine of unequal distribution in de Vry's subsequent writings.

Opponents of absolute (and only) Mendelian inheritance raise the question: Doesn't intermediate states of offspring (i.e. offspring of varying sizes) in the case of truly quantitative traits (such as individual size) prove that there is no Are there discontinuities?Doesn't it reveal that there are two types of genetic phenomena, Mendelian inheritance of discontinuous variation and other forms of inheritance of continuous variation?Wouldn't it be more important to explain the inheritance of continuous variation?Because continuous variation is the basis of Darwin's theory of gradual evolution.Due to the lack of a theory of quantitative genetics, a split occurred in evolutionary biologists, forming two opposing schools, generally known as the Mendelian school and the biostatistical school.However, the above two names are only applicable during the period from 1900 to 1906, while this debate started after the publication of Bateson's "Data for the Study of Variation" in 1894 and continued to the 1930s and 1940s. Comprehensive period.This debate created a deep divide in evolutionary biology that persisted through the first three decades of this century (Mayr and Provine, 1980).This is a conflict between two philosophical views. The Mendelian school advocates the idea of ​​essence and emphasizes the behavior of a single genetic unit, while the biostatistical school pays attention to population phenomena and is keen on holistic explanation.It can even be said that the divergence of these two opposites dates back to the eighteenth century.In fact fusion inheritance, one of these ancient problems, must be introduced before we can proceed to analyze what happened after 1900.

Naturalists and animal breeders have known since the eighteenth century that "mutations" (discontinuous variants) once appearing can remain unchanged for many generations.In contrast to this, when different species or different domesticated varieties and geographical varieties (geographical sects) are crossed, they are "fused" (blended originally means mixing, blending, mixing).Darwin, for example, used the word "merge" almost without exception in connection with interbreeding between species or breeds. So did Moritz Wagner and other naturalists after 1859 when they wrote about fusion. The word "fusion" is derived from the perfectly correct observation that there is very little perceptible Mendelian segregation in the F2 offspring of crosses between most species (see Chapter 14, Kerr Luther).It must be emphasized that all these scholars consider phenotypes, and since most differences between species are highly polygenic, the phenotypes of hybrids between species and breeds are generally intermediate, that is, they "fusion".When the word was originally used it referred to the phenotypic appearance.

Does this mean that these scholars also believe that the genetic factors of the observed phenotypic traits are also fused?They obviously do, but only partially believe it.For example, Darwin mentioned many times that the buds of the male parent and the female parent may either fuse at the time of fertilization, or they may just stick together and then separate.Darwin's particular emphasis on reverting mutation frequency completely refuted the notion that he believed in absolute fusion.In (1859) he mentions no fewer than eight reverting mutations, and a separate chapter (thirteen chapters) is devoted to it in Variations of Animals and Plants under Domestic Conditions (1868).In the second edition of the book (1893) he implicitly mentions that perhaps "it is better to say that the elements of the two parent species exist in a hybrid in a dual state: fused together or quite separate".Elsewhere he also refers to "pure" micrograsses and "hybrid" microbuds of hybrid progeny.Darwin also mentioned with particular admiration the idea of ​​Chunding's parental character being infused in hybrids (see Chapter 14).In his letter to Huxley in 1856 (M.L.D. 11:103), perhaps better than in all his published articles, he expresses his deep belief in granular inheritance: "I have been thinking lately (very roughly and Vaguely) true fertilized reproduction would be some form of admixture of two different individuals (or rather an infinite number of individuals, since each parent in turn has its parent and ancestors) rather than true fusion. I can't think of any There are other ideas that account for the reversion to such an extent in hybrid organisms to their ancestral forms."

It should be admitted that in his later writings Darwin never again emphasized the particle theory of heredity as much as he does in this letter, but he never adopted the absolute theory of fusion, as his opponents claim.De Vry (1889) correctly pointed out that Darwin's explanation of hereditary phenomena was more consistent with granular inheritance than fusion inheritance on the whole.Although Darwin was the author of the two-volume work on variation, his main interest was not in creating a theory of heredity, so he cited reverting mutations more as evidence of common ancestry than as evidence for a theory of heredity.He was particularly interested in the zebra stripes that often appear on the legs and shoulders of horses and donkeys, suggesting that he used them to support the theory of a common ancestor.

Nageli was one of the few biologists (possibly including Hertwick) who openly supported the theory of absolute fusion inheritance after Darwin, acknowledging the hypothesis of fusion inheritance and the use of microbuds, molecular clusters or other particles as genetic material are identical, as long as the paternal and maternal granules fuse with each other at fertilization.All others consider not only the granules to be carriers of heredity (some of which may of course fuse at fertilization) but also that at least some granules are passed intact from one generation to the next (e.g. Galton, 1876; de Vries, 1889).The assertion that Darwin and most scholars of variation prior to 1900 recognized absolute fusion inheritance (which I believe was first proposed by Fisher in 1930) has no basis in fact (see Ghiselin, 1969; Vorzimmer, 1970).This was clear at the time, as can be seen from a passage in 1898 by the American embryologist EG Conklin: "Many other phenomena, notably granule inheritance, independent variability of body parts, and latent and The genetic transmission of distinct characters can at present be explained only as ultramicroscopic units of structure" (cited in Carlson, 1966).

Considering the general acceptance before 1900 of the theory of granular inheritance, the theory that genetic elements transmitted from parents do not fuse after fertilization but maintain their integrity throughout the life cycle, it is said that 1900 rediscovered Mendel's laws The most important result of this is that it is quite wrong to say that granular inheritance replaces the accepted fusion inheritance.Many scholars (including Darwin) agree that it was a bit of both.I feel that the continued acknowledgment of fusion inheritance played only a small role in the resistance to Mendelism after 1900.R.A. Fisher's explanation and those who believed in it forgot that genotype and phenotype were not clearly distinguished around 1909, and the word "fusion" was traditionally used to indicate the intermediate state of phenotype ( especially in species crosses).It doesn't necessarily have to do with the behavior of the genetic material.

Another ambiguity before and earlier in Mendelianism must therefore be clarified.Important question, the difference between phenotype and genotype. The debate over fusion inheritance shows how important it is to distinguish genotypes (the overall genetic makeup of an individual) from phenotypes (the traits of individuals that change from genotypes during development). Among 19th-century biologists, almost exclusively, Gallon noticed this distinction.His new word "stirp" and redefined "heredity" clearly refer to hereditary type, and his vocabulary "nature vs. nurture" (nature and nurture, or heredity and environment) emphasizes this distinction.This question has been neglected, not only in Darwin's writings but also after Darwin. When the science of genetics was born in 1900, there was no distinction between the two, either in name or in concept, except Weismann's germplasm and constitution.In de Vry's view the individual as a whole is nothing more than a magnified image of the original set of panions in the nucleus of the fertilized egg (zygote).

This is why he never minded whether his word "mutation" referred to the phenotype or the germplasm on which it was based. But animal and plant breeders have long known that there is no genetic determinism such as de Vry's concept implies.There are many traits, such as tomato fruit size, that are both governed by genetic makeup and influenced by environmental factors. It was the Danish geneticist Wilhelm Johannsen (1857-1927) who first recognized the need for a distinction in terminology.Johnson's origin and education are highly unusual.He was largely self-taught, spending much of his early education in pharmaceutical and chemical laboratories.When he finally decided to turn to the study of plant physiology, he, like the Galton he admired, emphasized quantitative methods and statistical analysis.He was also an essentialist of earth making, and he found it difficult to understand the considerable variation in bean size in the bean after many generations of self-pollination, because the bean produced by the white flower pollination of many generations should be genetically Identical and mostly homozygous.In order to avoid this variation, he called the statistical average of the test samples "phenotype": "It is possible to call a statistically derived type ... simply called a phenotype ... a particular phenotype may be a certain This is an expression of biological unity, but it does not have to be. Most phenotypes found in nature by statistical studies are not!" Johnson's terminology, as well as his argument, make it clear that he is trying to achieve "pure Essence", so as to explore the "pure line".Later scholars found this typological definition useless and redefined phenotypes as actual traits of individuals. Although the name was coined by Johnson, the modern usage of genotype and phenotype is actually closer to Weismann's germplasm and constitution. After Johnson created the word "gene" (see Chapter 17), he combined the root word "(type)" with it to form the word "genotype", and its corresponding part is called "phenotype". "Genotype" or "hereditary type" refers to the genetic composition of a zygote formed by the union of two gametes: "We use the word genotype to refer to this genetic composition. This word does not rely on any hypothesis at all; it is Fact, not a hypothesis, that the different zygotes resulting from fertilization may be of a different nature, capable of forming phenotypically diverse individuals even under very similar circumstances of life" (1909: 165- 170).On the whole, however, Johnson wanted to consider the genotype as the genotype of a population or species (from the perspective of typology or schema). Woltereck (1909) also adopted a different term around the same time to express the important insight that the same genotype can produce quite different phenotypes under different environmental conditions.He believes that what is inherited through heredity is simply "reaction norms" (norm of reaction), that is, the quality of responding in a specific way to any environmental condition. However, the fundamental difference between genotype and phenotype was not really understood until the discovery (1944-1953) that the genotype (genotype) is composed of DNA and the body is composed of proteins (and other organic molecules).Even Johnson was not immune to confusion on this issue in the early days of genetics.The root cause of many important debates in the history of evolutionary biology is the inability to distinguish between genotype and phenotype, such as the essence of fusion inheritance and mutation.In fact, a full recognition of the difference between hereditary qualities (genotypes) and observable appearances (phenotypes) is necessary to ultimately negate soft inheritance.Johnson's own decisive contribution to the negation of soft inheritance was no accident, though it owed much to his accidental choice of a suitable test organism. Johnson chose a plant that could self-fertilize (self-fertilizing) the bean (Phaseolus vulgaris). Since plants of this species are usually selfed, they are highly homozygous.He selected 19 plants produced by self-crossing through multiple generations as the basic mother plants for selection.In each of these "pure lines" he selects the largest to smallest bean for breeding.Progeny variation was essentially the same in each experimental group, independent of the size of the bean on the mother plant. In other words, both bean and bean genotypes are the same in an inbred line, and the observed differences are phenotypic responses to different environmental conditions.The characteristic or important aspect of Johnson's experimental research is his accuracy in measuring thousands of beans and the thoroughness of the statistical analysis of the experimental results.He concluded that differences in bean size due to differences in cultivation conditions (fertilizer, light, moisture, etc.) cannot be passed on to the next generation.This conclusion is inevitable, no acquired traits are inherited.Because the phenotype is the result of the interaction of the genotype and the environment, it cannot be considered an accurate symbol of the genotype. Johnson's inbred experiments had a very peculiarly ambiguous effect on biology.On the one hand, it helped to weaken the influence of soft inheritance, which was still prevailing and popular at that time, but unfortunately, this experiment was cited by Johnson himself and other scholars as evidence of the invalidity of natural selection (see Chapter 12) . 18.1 Competing Theories of Heredity After clarifying the issues of genotype and phenotype and fusion inheritance, we can comprehensively consider the various reasons against the general application of Mendelian inheritance.A number of competing genetic theories at the time played a major role in the opposition.When Mendel's laws were rediscovered in 1900, these laws could not have occupied a blank field.Because of the fact that there were already some other theories of genetics (in particular, there were three main ones) that seemed to explain Darwin's gradual evolution better than Mendelianism. Darwin's cousin Galton (Francis Galton) continued to establish his previous system of genetic theory after 1875 (see Chapter 16).Among the early geneticists, only Galton paid attention to the genetic variation of the population.Unlike breeders and Mendelians, he paid special attention to quantitative traits such as height and skin color.He found that the mean values ​​of these traits in a population were generally the same from generation to generation.On average, the children of the tallest men were shorter than the average for the men and their spouses.That is, their offspring "revert" to the population mean.Conversely, the offspring of the shortest men regress upward to the population mean.Galton's reasoning fits well with common sense.He believed that each person received half heredity from his father and half from his mother.Applying the same reasoning to grandparents, each person inherits a quarter of hereditary qualities from his grandfather and grandmother, an eighth from his great-grandparents, and so on.Thus, the genetic contribution of the ancestors is halved every generation.This is what was later called Galton's Law of Ancestral Inheritance. At first glance, Galton's explanation of heredity seems to be far more complete than Mendel's law of segregation in terms of continuous variation.Darwinists such as Weldon, Pearson, etc., who subscribed to Darwin's concept of gradual evolution, supported Galton when forced to choose between discontinuous and continuous inheritance (although Galton himself believed in catastrophic evolution; see 10. Chapter II).Galton's theory of ancestral inheritance, although modified by Pearson, still has many shortcomings, one of which is that it is completely descriptive and does not actually provide any explanation of why, and another disadvantage is that it does not allow any prediction. Galton's worst mistake, however, was his statistical transfer of correct conclusions about the genotype as a whole to the mode of inheritance of individual traits.Although Galton admitted that particles were the material basis of hereditary phenomena (see Chapter 16), in his reasoning he treated these particles as if they were fused.Homozygous recessiveness from heterozygous parents (which in turn are derived from heterozygous progenitors) is completely unexplained by Galton's law and makes this law (or doctrine) subject to unanimous and categorical rejection. Galton's law is true in saying that individuals and their ancestors are likely to be similar, but it cannot be applied to individual genetic elements. But it will take a long time to fully realize this, and Mendelianism can only hope to be generally accepted after Galton's Law has been abandoned by all supporters. Even after Weldon died in 1906 and Pearson and Galton moved on to other areas of research, the question of the inheritance of continuous variation remained controversial.In fact, in a prophetic article by the British mathematician Yule (1902: 234-235), it was proposed that continuous variation may be caused by the joint action of multiple genetic factors, but this idea was completely ignored by his contemporaries. people value (see below). Trying to explain continuous variation in a non-Mendelian way took longer.W. E. Castle, one of the most gifted experimentalists in the early days of genetics, found that the white guinea pigs produced by crossing the white guinea pigs with the ancestor black guinea pigs were better than the white guinea pigs obtained from the pure line of white guinea pigs. The blackness on the limbs (and sometimes elsewhere) is darker and heavier.Based on these findings he later developed the so-called contamination theory, whereby the white genetic elements of heterozygotes are "contaminated" by the black genetic elements (and vice versa) during meiosis, so that their offspring display a slight degree of intermediate traits.This is the last "soft inheritance" theory proposed by a well-known geneticist.This interaction of either-or traits certainly helps to account for continuous variation to some extent, and the theory is therefore popular with Darwinists.Kessel's pollution theory caused an academic debate between him and Morgan and his students, especially Mill. Unable to confirm his prediction in 1919 in a decisive backcross experiment, Kessel abandoned his pollution theory.His idea was based on the early Mendelians' (particularly Bateson's) concept of unit traits, where each trait is controlled by a single, specific genetic factor.If characters change (as in Kessel's cross experiments), it must be the result of a change in the hereditary factor.The multifactor theory (see below) refers to the fact that several (if not many) genes can affect (modify) a single trait, thus making one abandon the unit trait theory. After Kessel's contamination theory was rejected, the last remaining theory tried to explain continuous variation in a non-Mendelian way.According to this theory, continuous variation is caused by a special "species substance" that may be present in the cytoplasm and has nothing to do with discrete Mendelian genes. The idea of ​​an unchanging species of material passed on from generation to generation was slowly replaced over a considerable period of time by the theory that inheritance is controlled by genes located on chromosomes.Numerous observational studies from the 1880s to the 1920s seem to suggest that these observations are better explained by the assumption that there is a fairly stable, unchanging, diffusible class of species-specific genetic material that may exist in In the cytoplasm and co-exist with genes on the chromosomes.According to this assumption, chromosomes are carriers of discontinuous traits (as exemplified by de Vry and Morgan's mutations) while continuous variation, as well as variation associated with the 'true essence' of a species, is carried by the cytoplasm.This view is popular among embryologists.Observations and experiments have repeatedly shown that the cytoplasm of the mature egg is intricately organized and appears to be the major control center of early development. Research in recent years has also fully confirmed this point.This fact is related to Roux's shift from equal split to indirect qualitative split.Only recently has it been discovered that this organization of the cytoplasm is controlled by genes while the egg is still forming in the ovary.In any event, from His (1874) to Jacques Loeb in 1916, many biologists were openly skeptical about the role of the nucleus at all in early development or in the essence of species.Boverly himself, while providing conclusive evidence for the important role of nuclear defense (see Chapter 17), continued to be conservative on this issue (1903, Rouxs Archiv, 16).He believed that species traits could be distinguished as those that could be explained by chromosomal inheritance, but the inheritance of those traits that assigned species to higher taxa was an unexplained problem.Before the 1930s, many biologists divided genetic phenomena into two categories controlled by the nucleus or the cytoplasm, respectively.Even the most orthodox Darwinist among geneticists on the continent, E. Baur also questioned whether the traits of higher taxa could be explained in the same way as the traits of species.Variation in these traits does not appear to be related to Mendelian inheritance. Proponents of cytoplasmic inheritance have some plausible reasons.Some scholars (such as Conklin and Guyer) who study the phenomenon of highly unequal cleavage specifically mention the obvious effect of egg cytoplasm in early embryogenesis.The naturalist noticed that mutations of the kind Morgan was studying, such as white eyes, luteal color, loss of bristles, missing wings, etc., occur not only in the common fruit fly (Drosophila melanogaster), but also in other species of fruit flies, so They claim that there is no evidence that these subtle traits that differentiate species are chromosomally inherited.Opponents of absolute chromosomal inheritance fail to understand how it is possible for so many heritable traits to exist on such a small chromosome. Winkler (1924) has given a good summary of the arguments in favor of cytoplasmic inheritance. In particular, botanists have discovered many phenomena that seem to require cytoplasmic inheritance in order to be explained. Wettstein (1926) suggested that the genetic material located in the cytoplasm be called "cytoplasmic gene" (Plasmon) to distinguish it from the "gene (group)" (genom) in the nucleus.Many botanists (especially German botanists) have discovered the genetic effect of cytoplasm, such as Collens (Mirage and other genera), Michaelis (Schwemmle), Schwemmle (evening primrose), Oehlker ( Coriolis), Wettstein (mosses), etc.In this context, Goldschmidt also explained some of his findings in the genus Cytoplasma.The reason why German scholars emphasize cytoplasm is obviously a continuation of the developmental phenomenon that German genetics research focused on in the 1880s and 1890s.It is too early to look back at the research on these cytoplasmic phenomena, and the time is not yet ripe. Therefore, although many scholars participated in the genetics research in Germany, its contribution to genetics of transmission is not equal to that of Bateson. Cuenot, Kessel or Morgan school, they deliberately avoided the issue of cytoplasmic inheritance. The idea of ​​a broadly important independent role of the cytoplasm for hereditary phenomena was eventually rejected in a number of different ways (Wilson, 1925).First, from a theoretical point of view: (1) The extreme precision in controlling the division of nuclear chromatin substances is unmatched by cytoplasmic division. (2) The contribution of the male parent and the female parent to the genetic composition of the offspring is basically the same, which has been confirmed by reciprocal cross hybridization experiments, although the cytoplasmic content of female gametes and male gametes is extremely unequal in many species.Boverly (1889) also proved this very well, when he fertilized (removed) (cell) nuclear fragments of large eggs of sea urchins of one genus with the sperm of another genus of sea urchins. traits, but the real hybrid embryos just show intermediate traits between the two genera. (3) The meiosis of the maturing female gamete (egg cell) only affects the chromatic substance, and does not affect the cytoplasm.In contrast, the developing sperm has very little cytoplasm, resulting in a large difference in cytoplasm content between the paternal and maternal parents, but the genetic material of the paternal and maternal parents is identical. Even more important than these theoretical considerations is the finding of an explanation for this seemingly exceptional phenomenon. "Delayed Mendelian inheritance" is one such exception. When there is an abundance of egg cytoplasm, the first steps in development are often controlled by factors in the egg cytoplasm that are of course products of the maternal individual.For example, the direction of rotation of a snail's shell texture is dextrorotary (clockwise) or levorotatory (counterclockwise) is determined by the egg cytoplasm during the first cleavage.But it was later shown that this direction of rotation is actually controlled by a gene acting on the ovarian egg before fertilization, and at least as far as the experimental material used for the classic study of this problem (Boycott and Diver, 1923) is concerned, Limnaea Peregra) dextrorotation is dominant.A left-handed female snail will produce a left-handed offspring after being fertilized by a right-handed male, but the latter will then produce a right-handed offspring, which is due to the influence of the dominant right-family paternal gene on the formation of the egg cytoplasm.Genetics textbooks contain many examples of this delayed Mendelian inheritance, sometimes over several generations, that at first glance appears to be cytoplasmic. A second class of phenomena cited as evidence of cytoplasmic inheritance is that the contents of plant cells (such as chloroplasts and other so-called plastids and organelles) transmit their characteristics more or less independently of the nucleus.Some of them actually have their own genetic material (DNA), which seems to have been formed along with their evolutionary origins.Leaf patterning is also a maternally inherited plastid trait in certain species of plants.Organelles in animal cells, such as mitochondria, also have their own DNA.But these phenomena fundamentally do not contradict the chromosome theory of inheritance. The same is true of the substantial autonomy of certain cytoplasmic structures in protozoans (ciliates) discovered by Sonneborn (1979). The third type of phenomenon that was once considered to be evidence of cytoplasmic inheritance is that some tissues are infected by microorganisms and then passed on to gametes when gametes are formed. The phenomenon of "petite colonies" discovered by Ephrussi (1953) in yeast, Sonneborn in The "Kaba factor" found in Paramecium (Pteer et al, 1974), the sex ratio factor in Drosophila, the sterile factor in house ants, etc. all belong to this category of phenomena. Thus one phenomenon after another that at first was seen as indicating the existence of cytoplasmic inheritance eventually had a gene-chromosomal explanation.The possibilities of cytoplasmic inheritance were finally clarified when the cytoplasm was able to be dissected into its component parts by electron microscopy and corresponding chemical studies.But that doesn't mean the end of the day for cytoplasmic genetics, which plays an important role both in development and in regulating gene activity.In fact there are already indications that the fine structure of the cytoplasm has a greater role than we currently know.It is also possible (if not with good reason to believe) that this structure is species-specific and related to many processes in the cell, and Sonneborn's research does support this idea.So the old view that the cytoplasm was genetically important was not completely wrong and abandoned, but it has been greatly revised. 18.2 The Mendelian Interpretation of Continuous Variation Since the non-Mendelian explanations of continuous variation have been proved to be invalid one after another, the inevitable conclusion is that continuous variation can only be explained by Mendelian discontinuous genes.This interpretation becomes possible when it is recognized that a single aspect of the phenotype may be controlled by genes located at several different loci.In fact this was already spelled out in detail by Mendel when explaining the results of some of his experiments on hybridization of species (such as the hybridization of Dwarfiaceae with Versicolor bean) and also by Gardenler's hybridization of species.Even Bateson realized that this might be the solution to the paradox: "If there were only a few, say, four or five pairs of alleles, the various combinations of nazygosity and heterozygosity, arranged in sequence, might yield a very For a nearly continuous curve, the purity (i.e., discontinuity) of its individual components, which are practically impossible to detect" (1906), since a single trait (such as length) is determined by two, Three or more genes are affected.He concluded that "discontinuous variation must be imperceptibly incorporated into continuous variation because of the composite nature of most of the traits studied." However, the inheritance of continuous variation by the same discrete Mendelian The conclusion that it was explained as discontinuous variation took a long time to be accepted by scholars who opposed Mendelianism. The Swedish plant breeder Nielsen-Earle was the first to use experiments to prove (1908-1911) that the quantitative traits produced by continuous variation could be inherited in the true Mendelian way.In a cross experiment of two wheat varieties with red and white seeds respectively he found plants with red seeds in F1 and F2.A very special segregation phenomenon occurred in F3 after self-pollination of F2 plants (see genetics textbook for details).His findings are consistent with the hypothesis that seed color is controlled by three separate genes that are inherited independently.It was later known that wheat is hexaploid, and Nelson-Earle happened to choose it as a research material because hexaploid contains three sets of chromosomes, each of which has a gene that controls red color.Later he also found some examples of non-polyploidy, where a single trait is controlled by two or three separate genes. East (191o) when studying maize and Davenport (1910) independently explained the same continuous variation when studying human skin color.It is now known that the number of individual genes that control a single trait can be very large.In addition, geneticists who study mice believe that the gene for skin color also plays a role in mouse size. The outstanding effect of multifactorial inheritance is that it can transform discontinuous variation of genotype into continuous variation of phenotype.For example, in the wheat studied by Nelson-Earle, the greater the number of red dominant genes, the darker the red.Different individuals in a wheat population can range from homozygous recessive for all red genes (thus no red color at all) to homozygous dominant for all three genes, so the red color varies continuously from light to dark.When non-genetic phenotypic variation is partially superimposed on it, a smooth continuous variation curve is formed, although this (continuous) variation is formed by individual, that is, discontinuous Mendelian factors.Thus at last the mystery of the genetic basis of continuous variation was solved.