Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 19
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The characteristic shown at the far right is the dominant traitthat appears in the hybrid produced by crossing.Page 35them from the pure-breeding parents, the P1 generation. Mendel also performed the reciprocal cross, in whichplants from the variety with round seeds were used as the pollen parents and those from the variety with wrinkledseeds as the female parents. As before, all of the F1 seeds were round (Figure 2.3). The principle illustrated by theequal result of reciprocal crosses is that, with a few important exceptions that will be discussed in later chapters,The outcome of a genetic cross does not depend on which trait is present in the male and which is present inthe female; reciprocal crosses yield the same result.Similar results were obtained when Mendel made crosses between plants that differed in any of the pairs ofalternative characteristics.
In each case, all of the F1 progeny exhibited only one of the parental traits, and the othertrait was absent. The trait expressed in the F1 generation in each of the monohybrid crosses is shown at the right inFigure 2.2. The trait expressed in the hybrids Mendel called the dominant trait; the trait not expressed in thehybrids he called recessive.Traits Present in the Progeny of the HybridsAlthough the recessive trait is not expressed in the hybrid progeny of a monohybrid cross, it reappears in the nextgeneration when the hybrid progeny are allowed to undergo self-fertilization. For example, when the round hybridseeds from the round × wrinkled cross were grown into plants and allowed to undergo self-fertilization, some of theresulting seeds were round and others wrinkled.
The two types were observed in definite numericalFigure 2.3Mendel was the first to show that the characteristics of the progeny produced by a cross donot depend on which parent is the male and which the female. In this example, the seeds of thehybrid offspring are round whether the egg came from the round variety and the pollen fromthe wrinkled variety (A) or the other way around (B).Page 36proportions.
Mendel counted 5474 seeds that were round and 1850 that were wrinkled. He noted that this ratio wasapproximately 3 : 1.The progeny seeds produced by self-fertilization of the F1 generation constitute the F2 generation. Mendel foundthat the dominant and recessive traits appear in the F2 progeny in the proportions 3 round : 1 wrinkled. The resultsof crossing the round and wrinkled varieties are summarized in the following diagram.Similar results were obtained in the F2 generation of crosses between plants that differed in any of the pairs ofalternative characteristics (Table 2.1).
Note that theTable 2.1 Results of Mendel's monohybrid experimentsParentaltraitsF1 traitround × wrinkled(Seeds)roundyellow × green(seeds)yellowpurple × white(flowers)purpleinflated ×constricted(pods)inflatedgreen × yellow(unripe pods)greenaxial × terminal(flower position)axiallong × short(stems)longNumber of F2progenyF2 ratio5474 round,1850 wrinkled2.96 : 16022 yellow,2001 green3.01 : 1705 purple,224 white3.15 : 1882 inflated,299 constricted2.95 : 1428 green,152 yellow2.82 : 1651 axial,207 terminal3.14 : 1787 long,277 short2.84 : 1first two traits (round versus wrinkled seeds and yellow versus green seeds) have many more observations than anyof the others; the reason is that these traits can be classified directly in the seeds, whereas the others can beclassified only in the mature plants.
The principal observations from the data in Table 2.1 were• The F1 hybrids express only the dominant trait.• In the F2 generation, plants with eitherthe dominant or the recessive trait are present.• In the F2 generation, there are approximately three times as many plants with the dominant trait as plants with therecessive trait.
In other words, the F2 ratio of dominant : recessive equals approximately 3 : 1.In the remainder of this section, we will see how Mendel followed up these basic observations and performedexperiments that led to his concept of discrete genetic units and to the principles governing their inheritance.Mendel's Genetic Hypothesis and Its Experimental TestsIn Mendel's monohybrid crosses, the recessive trait that was not expressed in the F1 hybrids reappeared inunchanged form in the F2 generation, differing in no discernible way from the trait present in the original P1recessive plants.
In a letter describing this finding, Mendel noted that in the F2 generation, "the two parental traitsappear, separated and unchanged, and there is nothing to indicate that one of them has either inherited or taken overanything from the other." From this finding, Mendel concluded that the hereditary determinants for the traits in theparental lines were transmitted as two different elements that retain their purity in the hybrids. In other words, thehereditary determinants do not "mix" or "contaminate'' each other.
Hence, a plant with the dominant trait mightcarry, in unchanged form, the hereditary determinant for the recessive trait.Page 37Figure 2.4A diagrammatic explanation of Mendel's genetic hypothesis to explain the 3 : 1 ratio ofdominant: recessive phenotypes observed in the F2 generation of a monohybrid cross.Note that the ratio of AA : Aa : aa genetic types in the F2 generation is 1 : 2 : 1.To explain his results, Mendel developed a genetic hypothesis that can be understood with reference to Figure 2.4.He assumed that each reproductive cell, or gamete, contains one representative of each kind of hereditarydeterminant in the plant.
The hereditary determinant for round seeds he called A; that for wrinkled seeds he calleda. Mendel proposed that in the true-breeding variety with round seeds, all of the reproductive cells would containA; in the true-breeding variety with wrinkled seeds, all of the reproductive cells would contain a. When thevarieties are crossed, the F1 hybrid should receive one of each of A and a and so should have the geneticconstitution Aa (Figure 2.4).
If A is dominant to a, the F1 seeds should be round.When an F1 plant is self-fertilized, the A and a determinants would separate from one another and be included inthe gametes in equal numbers. Hence, as shown in Figure 2.4, random combinations of the gametes should result inan F2 generation with the genetic composition 1/4 AA, 1/2 Aa, and 1/4 aa. The AA and Aa types should have roundseeds, and the aa types should have wrinkled seeds, and so the predicted ratio of round : wrinkled seeds would be3 : 1. (The genetic types AA, Aa, and aa can also be written with slashes as A/A, A/a and a/a, respectively; the twotypes of symbolism are equivalent.)The genetic hypothesis in Figure 2.4 also illustrates another of Mendel's importantPageConnection What Did Gregor Mendel Think He Discovered?Gregor Mendel 1866Monastery of St.
Thomas, Brno[then Brünn], Czech RepublicExperiments on Plant Hybrids(original in German)Mendel's paper is remarkable for its precision and clarity. It is worth reading in its entirety for this reason alone.Although the most important discovery attributed to Mendel is segregation, he never uses this term. His descriptionof segregation is found in the first passage in italics in the excerpt. (All of the italics are reproduced from theoriginal.) In his description of the process, he takes us carefully through the separation of A and a in gametes andtheir coming together again at random in fertilization. One flaw in the description is Mendel's occasional confusionbetween genotype and phenotype, which is illustrated by his writing A instead of AA and a instead of aa in thedisplay toward the end of the passage.
Most early geneticists made no consistent distinction between genotype andphenotype until 1909, when the terms themselves were first coined.Artificial fertilization undertaken on ornamental plants to obtain new color variants initiated the experimentsreported here.
The striking regularity with which the same hybrid forms always reappeared whenever fertilizationbetween like species took place suggested further experiments whose task it was to follow that development ofhybrids in their progeny. . . . This paper discusses the attempt at such a detailed experiment. . . .
Whether the plan bwhich the individual experiments were set up and carried out was adequate to the assigned task should be decided bya benevolent judgment. . . . [Here the experimental results are described in detail.] Thus experimentation alsojustifies the assumption that pea hybrids form germinal and pollen cells that in their composition correspond inequal numbers to all the constant forms resulting from the combination of traits united through fertilization. Thedifference of forms among the progeny of hybrids, as well as the ratios in which they are observed, find an adequatexplanation in the principle [of segregation] just deduced. The simplest case is given by the series for one pair ofdiffering traits.
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