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Ex12GeneticsProb - K101 Lab Exercise 12 Mendelian Genetics...

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K101 Lab Exercise 12 Mendelian Genetics: How are Traits passed from Generation to Generation? Genetics is the study of heredity - traits inherited from parent to offspring. Before ~1850 or so, long before the discovery of meiosis, chromosomes, or DNA, scientists thought that some fluid substance in the blood of animals or in the sap of plants was the hereditary material. The Blending Theory proposed that an offspring’s characteristics occurred by a "blending" of this hereditary fluid from the parents. If so, a white dog that mated with a black dog should produce only grey puppies, etc...clearly not the case! A different theory was put forth by Gregor Mendel in 1850. Mendel was an Austrian monk who was interested in plant breeding. He performed careful experiments with the garden pea, Pisum sativum , collected large amounts of data, and in doing so, was able to uncover the basic principles of genetic inheritance that still hold true today! Mendel proposed that traits are interited in discrete units (we now know these to be genes ). Mendel's experiments with the Garden Pea : Mendel's work started when he bred two types of pea plants - ones with purple flowers and ones with white flowers - that were true-breeding for flower color (meaning that the purple flowers produced only plants with purple flowers and the white plants produced only plants with white flowers). Mendel cross- pollinated the flowers. In the first filial (F1) generation, only purple flowers were seen. The white trait was masked. Mendel took this observation one step further, by allowing all of these purple flowered F1 plants to self- fertilize. He planted the resulting F2 seed. His Result: The white trait re-appears in the F2 generation in a ratio of 3 purple plants to 1 white. Mendel did this experiment with a total of 7 different traits, studying 22 strains of peas and always using large sample sizes, and he always saw a ~3:1 ratio in the F2 generation . Mendel realized that these results were explainable if three things were true. He hypothesized: 1. Every trait (like flower color, or seed shape, or seed color) is controlled by two " heritable factors ". [We know now that these are genes - we each have two copies of every gene. Different versions of the same gene are called alleles , such as the eye color gene alleles Brown and blue]. 2. If the two alleles differ, one is dominant (will be observed in the organisms appearance or physiology) and one is recessive (cannot be observed unless the individual has two copies of the recessive allele). Dominant traits mask the appearance of recessive traits. 3. Alleles are randomly donated from parents to offspring - the factors (alleles) separate when the gametes are formed, allowing all possible combinations of factors to occur in the gametes, and only one factor (allele) is present in each gamete.
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