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الموضوع: Genetics II

  1. #1
    عضو مميز الصورة الرمزية مالك محمد
    تاريخ التسجيل
    Aug 2011
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    افتراضي Genetics II

    Despite Gregor Mendel’s pioneering work on inheritance patterns, his 1866 publication, Experiments on Plant Hybridization, was dismissed or ignored by the scientific community for nearly 35 years. People failed to appreciate its rigor and its implications for scientific understanding of inheritance, breeding, بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى and cell biology. In 1900, Mendel’s work was rediscovered by three people, بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى , بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى , and بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى , who independently popularized and extended the studies. How could Mendel’s important work have been all but forgotten for nearly a generation?

    Perhaps some of the answer is that Mendel’s studies did not appear novel. Horticulturalists had long published plant-breeding experiments that superficially resembled those of Mendel. For example in the 1800s, بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى published nearly 100 articles on the subject of بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى , including an article about inheritance in pea plants, pre-dating Mendel’s work by three decades. Those studies, however, did not include rigorous backcrossing of the starting plants to generate pure-breeding parental strains, nor were the number of different colored peas in the offspring carefully recorded. When Mendel tried to repeat Knight’s experiments, he spent two years generating pure-breeding plants for each of the seven traits he followed, and then carefully counted plants with each trait after the parental plants were systematically crossed and then self-pollinated. Unlike Knight, Mendel employed a rigorous scientific strategy when devising his experiments. By assuring that he had pure-breeding plants to start, he made certain that his results were reproducible. In this way, Mendel’s work was quite different from that before him. Mendel’s decision to study discrete traits, to systematically cross-pollinate pure-breeding plants, and to keep count of all the offspring gave the reproducible 3:1 ratio of dominant to recessive traits, leading to what is now called Mendel’s First Law, the law of segregation.
    Another reason Mendel’s work was not celebrated in his lifetime is that the scientific community failed to appreciate the powerful reductionist approach that Mendel took to the complex question of inheritance. Holistic and organismal approaches were in style when Mendel was performing his experiments. Most notable at the time was Charles Darwin’s work, which examined biological specimens as whole entities, drawing conclusions from the sum total of their traits. However, Darwin’s work was criticized because it offered no mechanism for how بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى variations could arise. Unlike Darwin, بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى simplified the question he wanted to answer. Rather than asking “how do traits pass from بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى to their offspring,” he pursued a more scientifically testable question, namely “can I describe and predict how one trait is passed to the next generation?” The answers he got by teasing the question apart illuminated the solutions to the more complex questions of inheritance. Through his meticulous experiments with peas, Mendel’s work began to reveal how complex traits are passed on, and at the nexus of Mendel’s and Darwin’s observations is a model by which “descent with modification” could arise.
    The power of Mendel’s scientific approach can be seen in the work that led him to his Second Law, the law of independent assortment. The law of independent assortment states that different traits are inherited completely independently of one another. Yet how could Mendel have deduced this if he had no idea of the mechanism by which traits were inherited? When looking at two traits simultaneously, Mendel observed the ratio of dominant and recessive بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى for each trait, and discovered that the ratio of traits in the plants from such crosses arose as 9:3:3:1. Namely, 9 offspring showed both dominant traits, 3 offspring showed one dominant and one recessive trait, 3 offspring showed the complementary dominant and recessive mix and 1 of every 16 بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى showed both recessive traits. As an example, consider the experiment in which Mendel crossed a plant with yellow, round peas to a plant with green, wrinkled ones. Just as dictated by his First Law of Segregation, Mendel observed that all the F1 progeny from such a cross had yellow, round peas (the two dominant traits). Next Mendel self-pollinated these F1 progeny and counted 315 yellow round peas, 101 yellow wrinkled ones, 101 green round peas, and 32 green wrinkled ones in the F2 generation. Finding those rare peas with traits completely different from the peas of the F1 plant may have initially surprised Mendel, however the 9:3:3:1 ratio arose no matter which two traits he considered. What Mendel realized is that the mathematics behind the 9:3:3:1 ratio suggested independent inheritance. Consider two independent traits each governed by a dominant:recessive ratio of 3:1. If we cross those two ratios, the result of the cross is the 9:3:3:1 ratio that Mendel observed.
    A Punnett square (seen in Figure 1) can be used to understand these dyhibrid crosses. For the pure breeding plant with yellow and round peas, Mendel would have annotated the two dominant factors underlying these traits as YY and RR, respectively. By crossing this YYRR plant to a pure breeding plant with green, wrinkled peas, annotated yyrr, plants بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى for each factor (YyRr) would arise (the F1 generation). It does not matter which symbol is listed first, the presence of even one dominant factor gives rise to the dominant trait (see our بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى module). Since each F1 plant has one dominant and one recessive factor for each of the two traits examined, they are called “dihybrids.”
    Figure 1: Punnett square illustrating the cross of two independent traits (pea color=Y, and pea shape=R) in two purebred plants.
    Mendel produced the F2 generation by self-pollinating the F1 dihybrid plants. Based on a model of independent heredity, Mendel predicted that each of the traits in the dihybrid would be equally represented in the cross. Namely, 1/2 the plants would donate the dominant form of the color trait (Y);of these 1/2 would donate the dominant seed factor (R) and 1/2 would donate the recessive seed factor, thus resulting in either “YR” or “Yr “ crosses. Similarly, the recessive form of the color trait (y) could equally well pass to the F2 generation paired with either seed shape factor, so yR and yr should make up the other 1/2 of the possible combinations. In other words, the combinations of traits that could be mixed to form the F2 generation were: ¼ YR, ¼ Yr, ¼ yR, and ¼ yr. Writing these combinations along the top and side bars of a Punnet square (Figure 2) reveals how the بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى ratio of 9:3:3:1 arose in the F2 generation.
    Figure 2: Punnett square illustrating the cross of two independent traits in two dihybrid heterozygous plants.
    The outcome that Mendel observed from his dihybrid crosses confirmed that each trait could be described by a pair of factors that segregated to form بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى (his First Law), and further suggested that factors for multiple traits segregated independently thus forming the basis for Mendel’s Second Law of Inheritance.
    Mendel continued his methodical experiments to rigorously analyze his بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى . To test his ideas about random and independent segregation of dihybrid factors, he tested the prediction that the combinations of inputs from the F1 (dihybrid) generation were equally represented, namely four combinations existed: YR, Yr, yR, and yr. He tested this by crossing the dihybrids F1 plants with purebred plants that were doubly recessive for each factor: yyrr. Given that the purebred plant could donate only one possible بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى (yr), Mendel was able to test his بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى . The term used to describe such an experiment is a “testcross” and Figure 3 below shows the predicted outcomes from such a cross.
    Figure 3: Punnett square illustrating the cross of two independent traits in on purebred recessive trait plant (left) and one dyhybrid heterozygous plant (top).
    Confirming his ideas of independent assortment, the outcome of the dihybrid testcrosses exhibited a ratio of 1:1:1:1 of each بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى . The testcrosses powerfully supported Mendel's بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى regarding genetic contributions to the dihybrid cross and confirmed his notion that each factor in the dihybrid sorted independently.
    The rediscovery of Mendel’s work in 1900 allowed the بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى he described to be confirmed and extended. The scientists who rediscovered the research popularized Mendel’s principles by working with peas and other plants such as corn. Soon inheritance patterns in other بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى were investigated. By 1902, animals were shown (by William Bateson) to inherit traits in Mendelian fashion. Non-mendelian inheritance and بعد التسجيل عليك الرد بكلمة شكرا وعمل refresh للصفحة لرؤية المحتوى في المشاركة الاولى arising from multiple factors were later described, but the patterns that Mendel elucidated affected our understanding of heredity profoundly. Given that descriptions of the physical material of heredity (namely DNA) would not appear for years after Mendel’s work, his intuitive and insightful scientific analysis are all the more remarkable.

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  2. #2
    مشرفة الاقسام الاكاديمية الصورة الرمزية تمارا احمد
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  3. #3
    عضو مميز الصورة الرمزية مريم سماره
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    عضو فعال الصورة الرمزية omar david
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    افتراضي رد: Genetics II

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    عضو ذهبي الصورة الرمزية sajeda zaben
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  6. #6
    عضو مميز الصورة الرمزية ساره
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