Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 18
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Luzzatto. 1992. The molecular basis of glucose-6-phosphate dehydrogenasedeficiency. Trends in Genetics 8: 138.Watson, J. D. 1968. The Double Helix. New York: Atheneum.Page 30In this small garden plot adjacent to the monastery of St. Thomas, Gregor Mendel grew more than 33,500 pea plants inthe years 1856–1863, including more than 6,400 plants in one year alone.
He received some help fromtwo fellow monks who assisted in the experiments. Inside the monastery wall on the right is the Mendel museum(called the Mendelianum). The flowers are maintained as a memorial to Mendel's experiments.Page 31Chapter 2—Principles of Genetic TransmissionCHAPTER OUTLINE2-1 The Monohybrid CrossesTraits Present in the Progeny of the HybridsMendel's Genetic Hypothesis and Its Experimental TestsThe Principle of SegregationImportant Genetic TerminologyVerification of Mendelian Segregation by the Testcross2-2 Segregation of Two or More GenesThe Principle of Independent AssortmentDihybrid TestcrossesThe Big Experiment2-3 Mendelian Inheritance and ProbabilityMutually Exclusive Events: The Addition RuleIndependent Events: The Multiplication Rule2-4 Segregation in Human Pedigrees2-5 Genetic AnalysisThe Complementation Test in Gene IdentificationWhy Does the Complementation Test Work?Multiple Alleles2-6 Modified Dihybrid Ratios Caused by Epistasis2-7 Complications in the Concept of DominanceAmorphs, Hypomorphs, and Other Types of MutationsIncomplete DominanceCodominance and the Human ABO Blood GroupsIncomplete Penetrance and Variable ExpressivityChapter SummaryKey TermsReview the BasicsGuide to Problem SolvingAnalysis and ApplicationsChallenge ProblemsFurther ReadingGeNETics on the webPRINCIPLES• Inherited traits are determined by the genes present in the reproductive cells united in fertilization.• Genes are usually inherited in pairs—one from the mother and one from the father.• The genes in a pair may differ in DNA sequence and in their effect on the expression of a particular inheritedtrait.• The maternally and paternally inherited genes are not changed by being together in the same organism.• In the formation of reproductive cells, the paired genes separate again into different cells.• Random combinations of reproductive cells containing different genes result in Mendel's ratios of traits appearingamong the progeny.• The ratios actually observed for any traits are determined by the types of dominance and gene interaction.• In genetic analysis, the complementation test is used to determine whether two recessive mutations that cause asimilar phenotype are alleles of the same gene.
The mutant parents are crossed, and the phenotype of the progeny isexamined. If the progeny phenotype is nonmutant (complementation occurs), the mutations are in different genes;if the progeny phenotype is mutant (lack of complementation), the mutations are in the same gene.CONNECTIONSCONNECTION: What Did Gregor Mendel Think He Discovered?Gregor Mendel 1866Experiments on plant hybridsCONNECTION: This Land Is Your Land, This Land Is My LandThe Huntington's Disease Collaborative Research Group 1993A novel gene containing a trinucleotide repeat that is expanded and unstable on Huntington's diseasechromosomesPage 32Gregor Mendel's story is one of the inspiring legends in the history of modern science. Living as a monk at thedistinguished monastery of St.
Thomas in the town of Brno (Brünn), in what is now the Czech Republic, Mendeltaught science at a local trade school and also carried out biological experiments. The most important experimentswere crosses of sweet peas carried out from 1856 to 1863 in a small garden plot nestled in a corner of themonastery grounds. He reported his experiments to a local natural history society, published the results and hisinterpretation in its scientific journal in 1866, and began exchanging letters with Carl Nägeli in Munich, one of theleading botanists of the time. However, no one understood the significance of Mendel's work.
By 1868, Mendelhad been elected abbot of the monastery, and his scientific work effectively came to an end. Shortly before hisdeath in 1884, Mendel is said to have remarked to one of the younger monks, ''My scientific work has brought me agreat deal of satisfaction, and I am convinced that it will be appreciated before long by the whole world." Theprophecy was fulfilled 16 years later when Hugo de Vries, Carl Correns, and Erich von Tschermak, each workingindependently and in a different European country, published results of experiments similar to Mendel's, drewattention to Mendel's paper, and attributed priority of discovery to him.Although modern historians of science disagree over Mendel's intentions in carrying out his work, everyoneconcedes that Mendel was a first-rate experimenter who performed careful and exceptionally well-documentedexperiments.
His paper contains the first clear exposition of transmission genetics, or the statistical rules governingthe transmission of hereditary elements from generation to generation. The elegance of Mendel's experimentsexplains why they were embraced as the foundation of genetics, and the rules of hereditary transmission inferredfrom his results are often referred to as Mendelian genetics. Mendel's breakthrough experiments and concepts arethe subject of this chapter.2.1—The Monohybrid CrossesThe principal difference between Mendel's approach and that of other plant hybridizers of his era is that Mendelthought in quantitative terms about traits that could be classified into two contrasting categories, such as roundseeds versus wrinkled seeds.
He proceeded by carrying out quite simple crossing experiments and then looked forstatistical regularities that might suggest general rules. In his own words, he wanted to "determine the number ofdifferent forms in which hybrid progeny appear" and to "ascertain their numerical interrelationships."Mendel selected peas for his experiments for two reasons. First, he had access to varieties that differed inobservable alternative characteristics, such as round versus wrinkled seeds and yellow versus green seeds. Second,his earlier studies had indicated that peas usually reproduce by self-pollination, in which pollen produced in aflower is used to fertilize the eggs in the same flower (Figure 2.1). To produce hybrids by cross-pollination, heneeded only to open the keel petal (enclosing the reproductive structures), remove the immature anthers (thepollen-producing structures) before they shed pollen, and dust the stigma (the female structure) with pollen takenfrom a flower on another plant.Mendel recognized the need to study inherited characteristics that were uniform within any given variety of peasbut different between varieties (for example, round seeds always observed in one variety and wrinkled seedsalways observed in another).
For this reason, at the beginning of his experiments, he established true-breedingvarieties in which the plants produced only progeny like themselves when allowed to self-pollinate normally.These different varieties, which bred true for seed shape, seed color, flower color, pod shape, or any of the otherwell-defined characters that Mendel had selected for investigation (Figure 2.2), provided the parents for subsequenthybridization. A hybrid is the off-Page 33Figure 2.1Crossing pea plants requires some minor surgery in which the anthers of a flower are removedbefore they produce pollen. The stigma, or female part of the flower, is not removed. It isfertilized by brushing with mature pollen grains taken from another plant.spring of a cross between parents that differ in one or more traits. A monohybrid is a hybrid in which the parentsdiffer in only one trait of interest.
(They may differ in other traits as well, but the other differences are ignored forthe purposes of the experiment.)It is worthwhile to examine a few of Mendel's original experiments to learn what his methods were and how heinterpreted his results. One pair of characters that he studied was round versus wrinkled seeds. When pollen from avariety of plants with wrinkled seeds was used to cross-pollinate plants from a variety with round seeds, all of theresulting hybrid seeds were round. Geneticists call the hybrid seeds or plants the F1 generation to distinguishPage 34Figure 2.2The seven character differences in peas studied by Mendel.
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