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Mendel and heredity pdf

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  • 1. Mendel and Heredity
  • 2. Fundamentals of Genetics • List four characteristics that make Pisum sativum a good subject for genetic studies. • Outline the three major steps of Gregor Mendel’s garden pea experiments. • Explain how Mendel derived ratios from his observations. • Compare and contrast Mendel’s two laws of heredity.
  • 3. Gregor Mendel • Began experiments in 1857 • Austrian monk • Studied genetics of peas • Father of genetics • Failed his math/science teacher exam • Attended Univ. of Vienna - experimentation
  • 4. Why the Garden Pea? • Many varieties – 32 to start – used 7 • More than one form of the same trait • Small, easy to grow, matures quickly, large numbers of offspring… • Ability to cross-pollinate and self- pollinate
  • 5. Experimental Design • Step 1 – allow self-pollination to produce plants that are true- breeding. P generation. • Step 2 – cross-pollination. F1 generation. • Step 3 – self-pollination of F1 generation. F2 generatoin.
  • 6. Two ratios observed • Dominant trait – expressed trait in F1 generation. • Recessive trait – not expressed in F1 generation. • Self-pollination of F1 yields a 3:1 phenotypic ratio, but a 1:2:1 genotypic ratio.
  • 7. Mendel’s proposed Theory • Parent’s pass factors(genes) to their offspring that produce traits. • Two alleles for each trait – heterozygous (Tt) or homozygous (TT, tt) • Phenotype = physical appearance • Genotype = set of alleles • Dominant is expressed over recessive
  • 8. Mendel’s Laws • Law of segregation – member of each pair of alleles are separated when gametes are formed. • Law of Independent Assortment – pairs of alleles separate independently of one another during gamete formation.
  • 9. Interpreting Mendel’s Model • Capital letters= dominant alleles • Lower case letters = recessive alleles • Two alleles for each trait • Homozygous dominant = DD • Heterozygous dominant = Dd (hybrid) • Homozygous recessive - dd
  • 10. Probability • The likelihood that an event will take place. • = number of one kind of possible outcome total number of possible outcomes
  • 11. Monohybrid Crosses • Provide information about one pair of contrasting traits.
  • 12. Dihybrid Crosses • Involve two pairs of contrasting traits.
  • 13. Review Questions: 1. Write the formula used to determine the likelihood that an event will occur. How is probability used in genetics? 2. Construct a Punnett Square to predict the outcome of a monohybrid cross between two heterozygous tall pea plants. What are the expected phenotypic and genotypic ratios? 3. What is a dihybrid cross? What is the expected phenotypic ratio for a heterozygous dihybrid cross?
  • 14. 5 Factors that Influence Heredity #1 Incomplete Dominance • A trait is displayed that is intermediate between two parents. • Flower color in snapdragons – Red + White = Pink
  • 15. #2 Codominance • Two dominant alleles are expressed simultaneously • Different from incomplete dominance because both traits are displayed • Example) roan coat in horses is a result of a cross between a homozygous red and homozygous white horse…resulting in a horse with a coat of red and white hair.
  • 16. #3 Multiple Alleles • Genes with more than one allele • Example) three alleles can determine • blood type (A, B, and O) – A and B are dominant over O – Neither A nor B are dominant over each other – When A and B are present…they are codominant
  • 17. #4 Continuous Variation • Several genes influence a trait • Examples) height and weight • A variety of phenotypes exist from one extreme to another
  • 18. #5 Environmental Influences • Phenotypes being influenced by the environment • Example) the coat color of the arctic fox in winter
  • 19. Review Questions 1. What are dominant and recessive alleles? 2. Why were true-breeding plants important in Mendel’s experiments? 3. What is a Punnett Square? 4. What is the difference between Phenotype and Genotype? 5. Describe two inheritance patterns besides simple dominance.