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Fundamental principles governing the transmission of genetic traits, discovered by an Augustinian monk Gregor Mendel in 1856. Mendel performed his first set of hybridization experiments with pea plants. Although the pea plant is normally self-fertilizing, it can be easily crossbred, and grows to maturity in a single season. True breeding strains, each with distinct characteristics, were available from local seed merchants. For his experiments, Mendel chose seven sets of contrasting characters or traits. For stem height, the true breeding strains tall (7 ft or 2.1 m) and dwarf (18 in. or 45 cm) were used. He also selected six other sets of traits, involving the shape and color of seeds, pod shape and color, and the location of flowers on the plant stem.
The most simple crosses performed by Mendel involved only one pair of traits; each such experiment is known as a monohybrid cross. The plants used as parents in these crosses are known as the P1 (first parental) generation. When tall and dwarf plants were crossed, the resulting offspring (called the F1 or first filial generation) were all tall. When members of the F1 generation were self-crossed, 787 of the resulting 1064 F2 (second filial generation) plants were tall and 277 were dwarf. The tall trait is expressed in both the F1 and F2 generations, while the dwarf trait disappears in the F1 and reappears in the F2 generation. The trait expressed in the F1 generation Mendel called the dominant trait, while the recessive trait is unexpressed in the F1 but reappears in the F2. In the F2, about three-fourths of the offspring are tall and one-fourth are dwarf (a 3:1 ratio). Mendel made similar crosses with plants exhibiting each of the other pairs of traits, and in each case all of the F1 offspring showed only one of the parental traits and, in the F2, three-fourths of the plants showed the dominant trait and one-fourth exhibited the recessive trait. In subsequent experiments, Mendel found that the F2 recessive plants bred true, while among the dominant plants one-third bred true and two-thirds behaved like the F1 plants. See Dominance
Law of segregation
To explain the results of his monohybrid crosses, Mendel derived several postulates. First, he proposed that each of the traits is controlled by a factor (now called a gene). Since the F1 tall plants produce both tall and dwarf offspring, they must contain a factor for each, and thus he proposed that each plant contains a pair of factors for each trait. Second, the trait which is expressed in the F1 generation is controlled by a dominant factor, while the unexpressed trait is controlled by a recessive factor. To prevent the number of factors from being doubled in each generation, Mendel postulated that factors must separate or segregate from each other during gamete formation. Therefore, the F1 plants can produce two types of gametes, one type containing a factor for tall plants, the other a factor for dwarf plants. At fertilization, the random combination of these gametes can explain the types and ratios of offspring in the F2 generation (see illustration). See Fertilization, Gene
Mendel extended his experiments to examine the inheritance of two characters simultaneously. Such a cross, involving two pairs of contrasting traits, is known as a dihybrid cross. For example, Mendel crossed plants with tall stems and round seeds with plants having dwarf stems and wrinkled seeds. The F1 offspring were all tall and had round seeds. When the F1 individuals were self-crossed, four types of offspring were produced in the following proportions: 9/16 were tall, round; 3/16 were tall, wrinkled; 3/16 were dwarf, round; and 1/16 were dwarf, wrinkled. On the basis of similar results in other dihybrid crosses, Mendel proposed that during gamete formation, segregating pairs of factors assort independently of one another. As a result of segregation, each gamete receives one member of every pair of factors [this assumes that the factors (genes) are located on different chromosomes]. As a result of independent assortment, all possible combinations of gametes will be found in equal frequency. In other words, during gamete formation, round and wrinkled factors segregate into gametes independently of whether they also contain tall or dwarf factors. See Gametogenesis, Meiosis
It might be useful to consider the dihybrid cross as two simultaneous and independent monohybrid crosses. In this case, the predicted F2 results are 3/4 tall, 1/4 dwarf, and 3/4 round, 1/4 wrinkled. Since the two sets of traits are inherited independently, the number and frequency of phenotypes can be predicted by combining the two events:
This 9:3:3:1 ratio is known as a dihybrid ratio and is the result of segregation, independent assortment, and random fertilization. See Genetics
the principles of heredity that laid the foundations of genetics, based on the discovery and confirmation in 1900 of the forgotten work of G. Mendel (1866).
Just as Mendel’s discovery grew out of a long history of experiments with plant hybrids, the rediscovery and acceptance of the principles he formulated would not have been possible without advances in the study of cell division, fertilization, and development. In the words of N. I. Vavilov,“Mendel’s theory and its subsequent elaboration is one of the shining chapters of modern biology. Left in the shadows for almost half a century, under new conditions his theory illuminated and continues to illuminate a vast area of facts. It stimulated the unlimited accumulation of factual material in biology and simultaneously led to major generalizations, equally applicable to plants and animals, including man” (Izbr. trudy, vol. 5, 1965, p. 338).
Mendel’predecessors Conjecture about the laws of heredity began as early as the 18th century with the work of the first plant hybridizers. For example, with his interspecific plant crosses (1760-98), J. Kolreuter observed uniformity of traits in first-generation hybrids and the appearance of parental forms in succeeding generations. However, Kolreuter wrongly interpreted these phenomena as evidence of a gradual return to the original parental species, which he considered to be unchanged. Many cases of the “disappearance”of traits in the offspring of hybrids and of their reappearance in succeeding generations were described around the turn of the 19th century by such English horticulturists as T. E. Knight, who, like Mendel later on, studied hybrids of the sweet pea. The French plant breeders A. Sageret and C. Naudin came closest to understanding the phenomena of dominance, uniformity, and segregation. In 1825, Sageret found that the traits of cucurbit hybrids neither mix nor disappear but rather combine freely in succeeding generations. Sageret postulated the presence of a special “rudiment”for each trait, capable of either becoming manifest or remaining dormant.
Using interspecific crosses of fruit trees, Naudin formulated the theory (1861-65) that the “essences”responsible for opposite traits are incorporated into all of the cells of individuals of the first hybrid generation. The formation of the germ cells that produce the following generations would involve a “disjunction of essences,”as a result of which the original parental forms would reappear in pure form. Naudin restricted himself to spot-checking types of offspring; he was therefore unable to make an exact quantitative formulation of the principle of segregation.
Mendel’s discovery Mendel achieved clear-cut results because of his skillful selection of the forms to be crossed (pure pea varieties differing in only a few strictly defined traits, such as shape and color of the peas) and because he took into account all hybrid types appearing in the offspring. In contrast to the prevailing ideas of “fused”heredity, Mendel showed that the hereditary “elements” (factors) divide up and neither fuse nor disappear as a result of crossing. When two organisms differing in two pairs of contrasting traits (for example, smooth or wrinkled, green or yellow peas) are crossed, only one of each pair of traits (the “dominant,”as Mendel called it) will appear in the next generation. However, the trait that “disappeared” (the “recessive,”according to Mendel) will reappear in following generations.
Mendel experimentally demonstrated the constancy and mutual independence of the hereditary factors responsible for these traits and closely traced their fate and numerical ratios for all types of crosses. In addition, he offered an explanation for the qualitative and quantitative principles he had observed.
Using letters as symbols (A for smooth seeds, a for wrinkled, B for yellow seeds, b for green), Mendel showed that his quantitative principles could be explained only under the assumptions that (1) the hereditary elements joined in crossing separate again, in the germ cells of the hybrid; (2) when the hereditary elements separate, all possible types of germ cells are formed in equal numbers (50 percent A and 50 percent a; 50 percent B and 50 percent b ); and (3) in fertilization, the various germ cells combine according to the laws of chance, with equal probabilities for all possible combinations (A + A, A + a, a + A, a + a, B + B,B + b, b + B, b + b ).
The striking phenomenon that “disappearing” (recessive) traits reappear in the offspring and in definite numerical ratios was thus explained for the first time. For example, when two hybrid forms are crossed, or when a hybrid pollinates itself (Aa X Aa or Bb X Bb ), all three possible types reappear in the ratios (1AA : 2Aa : 1aa) and (1BB: 2Bb : Ibb). Constancy, independence, and free combination were demonstrated by Mendel for each pair of traits investigated (A—a, B—b, C—c)
Mendel also studied the numerical patterns of combination when forms differing in two or more pairs of traits are crossed. The results he obtained could not be explained except by the assumption that not only the individual hereditary elements responsible for each pair of traits but also the elements of different pairs can combine with complete independence (see Figure 1). This assumption led Mendel to formulate the law of combination of differentiating traits, according to which hereditary elements can enter into all combinations conceivable according to the rules of combination. Mendel conjectured that the processes that take place during the formation of germ cells are the basis of these principles: ’There are as many different germ cells possible as there are possible combinations of formable elements.”
Although Mendel was far ahead of his time in these conclusions, he could not, of course, completely understand the mechanism governing the realization of his principles in the germ cells. Scientifically, biology was not mature enough to appreciate Mendel’s discoveries until the beginning of the 20th century, when his work was not only rescued from oblivion but also substantiated experimentally. Study of the principles of inheritance in plants and animals (including man) gave rise to the rapidly developing field of Mendelism, the foundation of genetics.
Principles of Mendelism The initial phase in the development of Mendelism was marked by many conflicting interpretations of the number and nature of Mendel’s laws. The ideas put forth were therefore greeted with skepticism and criticized by the supporters of other approaches in biology. Much effort was ex-pended on attempts to discredit Mendel’s first law, the phenomena of dominance and recessiveness. The discovery of other manifestations of inherited traits (intermediate strains, succession of dominance, differential dominance) was regarded as a serious argument against Mendelism.
It is now clear that the principles of the transmission and distribution of hereditary factors (indeed, the main discovery of Mendel and those who continued his work) are completely un-related to the phenomena of dominance and recessiveness and are unshaken by the existence of great diversity in the manifestation of traits.
The first Mendelians, like their many critics, did not differentiate clearly enough between the concepts of hereditary trait and hereditary factor. The introduction of the terms “gene,”“geno-type,”and “phenotype”by the Danish scientist W. Johannsen in 1909 was therefore of decisive importance. Analysis of the differences between genotypic and phenotypic phenomena, based on Johannsen’s concept of pure lines, played a major role in the development of Mendelism. The principles of heredity could be understood clearly only on the basis of modern concepts of cell division and germ-cell maturation and only after the chromosomal theory of heredity had been substantiated.
The American scientist W. Setton (1902) and the German scientist T. Boveri (1902-07) showed that Mendel’s principles could be explained by the separation and combination of chromosomes during gametogenesis and fertilization. Thus, the law of segregation applies to alternative traits, which were later called alleles. Allelic traits are determined by the heritable factors, or genes, on a homologous pair of chromosomes. When the germ cells mature, each pair of chromosomes joined together during fertilization separates in such a way that each chromo-some, with one of the two allelic genes, enters a separate germ cell, or gamete. The gametes formed therefore contain just one of each of the types of allelic genes received by the hybrid from the parents. Both allelic genes never enter the same gamete, a phenomenon that the English geneticist W. Bateson called gamete purity.
Free combination during the fertilization of all types of germ cells results in the realization in the offspring of all possible combinations of genes. The independent combination of nonallelic genes is possible because such genes are found on different pairs of chromosomes. The phenomena of segregation, occurring simultaneously and independently in all chromosome pairs, en-sure all possible combinations of nonallelic genes.
It soon became evident that the number of nonallelic genes possessed by a given plant or animal species must exceed the characteristic number of chromosome pairs. Therefore, nonallelic genes found on the same pair of chromosomes must be inherited together. Instances violating the principle of the random combination of nonallelic genes were found well before a cytological explanation was proposed. The American geneticist T. Morgan and his co-workers showed in 1911 that every chromosome carries many genes. However, the genes (even those found on the same chromosome) may, in a certain percentage of cases, separate and combine independently—that is, the “linkage”is less than 100 percent. This linkage is disrupted by a special process called crossing over, which results in the ex-change of genes by homologous chromosomes. Thus, the combi-nation of nonallelic genes of the same chromosome or pair of chromosomes is regulated by the principles of linkage and crossing over.
General concepts concerning the relation of genes to the traits determined by them have also evolved considerably. Mendel and the early Mendelians tended to identify the gene completely with the trait, hoping to break down every organism into a sum of completely independent traits, equal in number to the genes present. It was later found that a single gene could determine a set of traits, and, conversely, that each trait depends on several genes. Thus, only the genes are separate and independent in inheritance, whereas a trait should be considered not a mosaic of separate entities but a whole that results from development under particular environmental conditions. The study of the complex patterns of development of hereditarily determined traits constitutes the subject matter of the independent science of phenogenetics. Advances in genetics—in particular, the discovery at the molecular level of the mechanisms of heredity— have finally consolidated Mendelism as a theory for the principal patterns of inheritance.
Mendelism and Darwinism Mendelism and the mutation theory that evolved early in the 20th century at first aroused antagonism between orthodox Darwinists and the Mendelians. For example, H. De Vries believed (1901-03) that “progressive mutation”alone could account for the origin of a new species. J. Lotsy (1912-13) advanced an unsubstantiated theory that genes remain unchanged and constant in number. Based on the presence and absence theory, Bateson (1914) maintained that genes can be lost, just as they can recombine. Neither of these theories took into account the fact that the principles of inheritance alone cannot explain the evolutionary process.
Many Darwinists, on the other hand, wrongly believed that evolution can take place only on the basis of uninterrupted minor changes on a mass scale. They greeted Mendelism and the mutation theory with hostility and denied the universality of the principles formulated by these scientists. British Darwinists (A. Wallace, E. Ray Lancaster) were sharply opposed to Mendelism. Relying on the tenets of the biometric school, they denied the continuous nature of hereditary variability, discrete alternative heredity, and the possibility that occasional mutations can be preserved with random crossing. The views of the orthodox British Darwinists also influenced certain Russian scientists, including K. A. Timiriazev and M. A. Menzbir. Nevertheless, Timiriazev understood that Mendelism “serves solely to support Darwinism by overcoming one of the most important objections raised against it” (Soch. , vol. 7, 1939, p. 236). The opponents of Mendelism in the USSR, too, failed to take this into consideration.
According to the notion of inheritance that prevailed before Mendel’s time, the traits of crossed organisms fuse. Thus, every new random trait should have no chance of surviving in the masses of forms of the species that differ from it. It was held, therefore, that natural selection was without effect in preserving a trait, even where useful in the struggle for existence. Mendelism made it possible to put aside the notion of fused heredity and, with it, the objections to the theory of natural selection. A newly appearing hereditary trait need not be manifested in the next generation after crossing; however, this does not mean that the hereditary factor responsible for it has been either fused or forever “absorbed”in the population. The traits determined by recessive genes found in the heterozygous state may reappear following a return to the homozygous state after a number of generations. These ideas, which proceeded naturally from Mendelism, were substantiated theoretically and experimentally by S. S. Chetverikov and his co-workers in 1926. The English scientists J. B. S. Haldane (1924 and after) and R. A. Fisher (1928-30) and the American scientist S. Wright (1931) independently elaborated the principles of evolutionary genetics. By the 1930’s, genetics, with Mendelism at its foundation, had become the acknowledged basis of modern Darwinism.
Thus, Mendelism has played a revolutionary role in biology, demonstrating that hereditary factors have a discrete, corpuscular nature and that their passage from generation to generation is determined by the principles of statistical variation. These new principles have made it possible to solve some of the difficult questions confronting Darwinism and to formulate the modern theory of microevolution. Mendelism has became the theoretical basis of modern methods of breeding microorganisms, farm crops, and domestic animals, and it has contributed to the development of medical genetics.
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A. E. GAISINOVICH