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11-1 The Work of Mendel What does every living thing inherit from their parents? Genetics – the study of heredity Look around at your classmates and make a list of some of the traits that are inherited.
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What did Mendel already know… Each flower produces pollen (sperm) and egg cells Cross fertilization (sexual) – male and female cells join Self-pollination (asexual) – pollen fertilizes eggs from same plant Mendel’s pea plants were true-breeding –A tall plant with green seeds would produce a tall plant with green seeds
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Genes, Alleles and Dominance Trait – specific characteristic –Mendel studied 7 different pea plant traits What are some examples? Plant Height, Seed Shape, Pod Color
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Genes, Alleles and Dominance Genes – the chemical factors that determine traits (the segment of DNA) ex. pea plant: height Alleles – different forms of a gene from each parent ex. pea plants: tall and short (T or t)
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Principle of Dominance Principle of dominance: some alleles are dominant and others are recessive Dominant allele – the trait is always shown –Capital letter (T = tall) Recessive allele – the trait that will only show if there is no dominant allele –Lower case letter (t = short)
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Heterozygous – organisms that have 2 different alleles for the same trait –Hybrid for that trait –Ex: Tt Homozygous – organisms that have 2 identical alleles for a particular trait True-breeding for a particular trait –Ex: TT or tt
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Genes, Alleles and Dominance Phenotype – physical characteristics –Ex: tall, short, yellow, green Genotype – genetic make-up –Ex: TT, Tt, tt Tall plants have the same phenotype (tall), but not the same genotype (TT or Tt) Why are TT and Tt genotypes for tallness, but tt is not???
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Mendel crossed plants to study offspring P generation – (parents, originals) – Tall x short F1 generation – (offspring of P) tall x tall F2 generation – (offspring of F1) 3 tall, 1 short
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11-2 Probability and Punnett Squares Mendel realized…the principles of probability could be used to explain the results of genetic crosses.
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Genetics and Probability Probability – the likelihood a particular event will occur. –Ex: probability of flipping a coin to heads = ½ or 50% –Probability of head 3 times in a row = ½ x ½ x ½ = 1/8 –The greater the number or trials, the closer to the expected ratio –Past outcomes do not affect future outcomes Alleles segregate randomly (like a coin)
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Probabilities Predict Averages To get an accurate prediction of flipping a coin – the coin should be flipped many times and an average taken. In genetics …the more offspring you get, the closer to the predicted ratio.
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Principle of Segregation Segregation = separation The alleles for tall vs. short separate during the formation of gametes – sex cells Each gamete carries one allele for each gene
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Principle of Independent Assortment Mendel questioned how alleles segregate Does it happen independently? Ex: Does the seed shape gene influence the seed color gene???? INDEPENDENT ASSORTMENT Need to follow 2 diff. alleles from one generation to the next.
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The Two-factor Cross: F1 4 alleles (2 genes) at the same time Provides the hybrid plants (F2 generation) Crossed a homozygous RRYY (round yellow peas) with a homozygous rryy (wrinkled, green peas)
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The two factor cross: F2 F1 – yields all offspring heterozygous for both traits F2 – heterozygous parents crossed F2 – yields 9:3:3:1 ratio
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Mendel discovered the principle of Independent assortment – genes segregate independently during the formation of the gametes –Helps account for many genetic variations in organisms –Seed shape & color gene do not influence each other
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Meiosis
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Ttrr x ttRr
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ttRr x TtRr
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