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Chapter 11: Introduction to Genetics
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Chapter 11: Introduction to Genetics

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  1. Chapter 11:Introduction to Genetics Section 11-2: Applying Mendel’s Principles

  2. Introduction • We cannot predict the future – If a parent carries 2 different alleles for a certain gene, there is no way to be sure which allele will be inherited by its offspring • The only thing we can do is predict the odds by applying Mendel’s principles

  3. Probability • Mendel was very careful to categorize and count all of the offspring each time he preformed a cross • Whenever he crossed 2 plants that were hybrid for height (Tt) about ¾ of the offspring were tall and ¼ were short • Mendel realized that he could apply the rules of probability to explain his results • Probability is the likelihood that a particular event will occur • Think about a coin flip – the probability of flipping heads is the same as the probability of flipping tails – ½ or 50% • If you flip a coin three times in a row, the probability that it will land heads up every time is ½ x ½ x ½, or 1/8 • Each flip is an independent event - past outcomes do not effect future outcomes

  4. Using Segregation to Predict Outcomes • The way in which alleles segregate during meiosis is just as random as flipping a coin, therefore the rules of probability can be used to predict the outcomes of crosses • Let’s go through Mendel’s cross: • A true-breeding tall parent (TT) was crossed with a true-breeding short parent (tt) • Each F1 offspring received a T and a t – meaning they were all Tt, and tall • When the F1 plants make gametes, ½ of the gametes will receive the T allele and the other ½ will receive the t allele

  5. Using Segregation to Predict Outcomes • The only way to get a short F2 offspring is for 2 t alleles to combine • The probability of 2 gametes coming together that both carry the t allele is ½ x ½, or ¼ • You would predict ¼ offspring to be short, and the other ¾ to be tall • This 3:1 ratio of dominant to recessive plants appeared again and again, consistently for each of the seven traits

  6. Using Segregation to Predict Outcomes • The reality is that not all organisms that look the same carry the same alleles – for example a plant that is tall could have two T alleles (TT) or is could have one T and one t (Tt) • All short plants have 2 t alleles (tt) • An organism that has two identical alleles for a gene – for example TT or tt – is said to be homozygous • Organisms with 2 dominant alleles are homozygous dominant • Organisms with 2 recessive alleles are homozygous recessive • Organisms that have two different alleles for a gene - for example Tt – are said to be heterozygous

  7. Probabilities Predict Outcomes • Probabilities are used to predict the outcomes of large events • In genetics, the larger the number of offspring the closer the results will be to predicted outcomes

  8. Genotype and Phenotype • An organism’s genotype is its genetic composition – the alleles it carries for each gene • It can be homozygous dominant, heterozygous, or homozygous recessive • An organism’s phenotype is its physical appearance, or how it looks • It can show the dominant phenotype or the recessive phenotype

  9. Genotype and Phenotype • Genotype determines phenotype Genotype: HD (TT) Phenotype: Dominant (tall) Genotype: Het (Tt) Phenotype: Dominant (tall) Genotype: HR (tt) Phentoype: recessive (short) • The genotype of an organism is inherited • Phenotype can also be influences by environment

  10. Using Punnett Squares • Punnett squares are simple diagrams used to predict the results of genetic crosses • Used to predict genotype and phenotype possibilities of offspring using probability • Refer to sheet for how to make PS for one factor crosses

  11. Independent Assortment • So far, we have only looked at one trait (1 gene) – called monohybrid crosses • Remember that Mendel also experimented to see what would happen if he followed two traits at the same time (2 genes) • These experiments are called dihybrid crosses

  12. Dihybrid Cross: F1 • Mendel crossed true-breeding plants with round, yellow peas with true-breeding plants with wrinkled green seeds • One plant HD for both traits – RRYY • One plant HR for both traits – rryy

  13. Dihybrid Cross: F1 • All of the F1 offspring produced round yellow peas • Punnettsquare shows that the genotype of each F1 offspring was RrYy, heterozygous for both seed shape and seed color

  14. Dihybrid Cross: F2 • Mendel used 2 F1 plants to create the F2 generation • Mendel observed that 315 of the F2 seeds were round and yellow, while another 32 seeds were wrinkled and green—the two parental phenotypes • But 209 seeds had combinations of phenotypes, and therefore genotypes, that were not found in either parent • The alleles for seed shape segregated independently of those for seed color • Genes that segregate independently do not influence each other’s inheritance – The Law of Independent Assortment • Refer to sheet for how to make PS for two-factor crosses

  15. Summary of Mendel’s Principles • The inheritance of biological characteristics is determined by individual units called genes, which are passed from parents to offspring • Where two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive • In most sexually reproducing organisms, each adult has two copies of each gene that segregate from each other when gametes are formed • Alleles for different genes usually segregate independently of each other