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KEAN BIO 1000 - Genetics

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GeneticsINTRODUCTIONMendelian inheritanceSample mating to illustrate simple Mendelian inheritancePod color exhibits simple Mendelian inheritanceDihybrid inheritanceA cross involving three allelesNumberGroupPercentageCONCEPTUAL LIFE SCIENCEGeneticsINTRODUCTIONLiving organisms resemble their parents. This is due to the transmission of traits or genetic characters from one generation to the next. We know from our study of reproduction that sexual reproduction involves receiving genetic material from each parent. In humans, half of your genetic material comes from your mother and half from your father. We know that the genetic material is DNA. The DNA is organized into genes thatare located on the chromosomes in the nucleus of the cell. Historically, the understandingof the transmission of the traits came before the understanding of the mechanism of the transmission. Gregor Mendel worked on traits with his peas in the mid-19th century. The chromosome was not discovered until the 1890s. Once biologists put the traits and the chromosomes together, we began to understand the science of genetics.CLASSICAL MENDELIAN INHERITANCEGregor Mendel was the abbot of a Catholic monastery in central Europe in the 19th century. One of his duties in the monastery involved tending the garden where he became interested in the traits of the pea plants. Table 19-1 lists some of the characters ofhis peas.Table 19-1. Characters in Mendel’s garden peas.Character AppearanceFlowers – color Red or whiteFlowers – location Axial or terminalPods – color Green or yellowPods – structure Inflated or constrictedSeeds – appearance Round or wrinkledSeeds – color Green or yellowPlants – height Tall or shortSome notes about Mendel’s peas. In Mendel’s day there was much more variation in plants that are used for food than you find today. Go to a store where they sell garden seeds and see what they have to offer. You will probably be able to buy seeds for peas that give round, green peas; in inflated, green pods; with white flowers that are terminal (on the ends of the branches). There are yellow string beans, the so-called “wax beans.” People generally expect that peas are green and are found in green pods. Mendel also had yellow peas and yellow pods. Just about the only variation found in modern days is the height of the plants.19-119-2Mendelian inheritanceWe begin studying Mendelian inheritance with red and white flower color. Mendel noted that the peas in his garden from year to year had red and white flowers. Some plants with red flowers came from plants with red flowers and made plants next year with red flowers. These were pure-breeding red-flowered plants. Similarly he noticed that he had pure-breeding white-flowered plants. Mendel kept very detailed, meticulous notes on his work with his peas. However, he had no clue as to the mechanism by which the characters were transmitted. He might have thought that it was like mixing cans of paint together.The flower is the reproductive structure of the plant. Figure 19-1 shows the parts of the flower. The female component of the flower is called the pistil. The male parts arecalled the stamens.Figure 19-1. Reproductive structures of the flower.The male stamens produce the pollen. The pollen grains are the male sexual units of the plant. They are produced in the anther of the flower that is supported by a filament. In the female pistil is an ovary, which contains ovules. The ovules are the female sexual units of the plant. Each ovule contains an egg that will become fertilized by the pollen. After fertilization, each ovule will become a seed while the ovary becomes a fruit.19-3When plants reproduce, pollen from the anther of one flower is transferred to the stigma of another flower. The pollen grains digest their way through the style to the ovary. In the ovary, chromosomes from one of the pollen grains fertilize each ovule. Sometimes, flowers can self-pollinate by transferring pollen from the anthers to the stigma in the same flower.Crossing plants involves transfer of pollen. A cross involves transfer of pollen from the stamens of one flower to the pistil of another. For example, pollen from a plant producing red flowers might be placed on the stigma of a plant with white flowers. Or, pollen from a white-flowered plant could be used to inoculate the stigma of a red-flowered plant. In either case, this constitutes a cross between a red-flowered plant and a white-flowered plant. Mendel did hundreds of these crosses over a period of many years.Once the cross has been performed, you need to wait for the peas to develop in their pods. Then you must harvest the peas and put them away in storage over the Winter.Then, next Spring, you plant the seeds and see what grows. So, as we discuss the types of crosses that Mendel performed with his peas, keep in mind that each stage in the cross takes one year.Sample mating to illustrate simple Mendelian inheritanceMendel took red-flowered peas that were pure-breeding and crossed them with pure-breeding white-flowered plants. These plants were the parental generation, represented by P. The following year, the plants that came up all had red flowers. None of the plants had white flowers even though one of the parents had white flowers. This is the generation of offspring and is called the F1 generation. The letter “F” comes from theadjective “filial” derived from a Latin word referring to children (offspring) and their relation to each other (the “sons” of the parental generation). This cross and its result is illustrated in Figure 19-2.Figure 19-2. Parental cross in simple Mendelian inheritance.19-4Crossing of the F1 generationThe following year, Mendel crossed the F1 red-flowered plants from the previous year. Recall that all of the F1 plants had red flowers even though one of the original parents had white flowers. The result of crossing the red-flowered F1 plants the production of an F2 generation. In the F2 generation, the white trait returned. Mendel also noticed that there were about three times as many plants with red flowers as there were with white flowers. See Figure 19-3. Figure 19-3. F1 cross and F2 results. After not being expressed in the F1 generation, white was expressed in the F2 generation.Pod color exhibits simple Mendelian inheritanceFor another example we can consider pod color. Mendel crossed pure-breeding plants having


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KEAN BIO 1000 - Genetics

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