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Chapter 14: Mendel & the Gene Idea

Chapter 14: Mendel & the Gene Idea. AP Biology. Gregor Mendel. Known as the “Father of Genetics” Experimented with pea plants to develop principles of genetics Peas exhibit complete dominance (he didn’t know this) Easily manipulated. Gregor Mendel.

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Chapter 14: Mendel & the Gene Idea

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  1. Chapter 14: Mendel & the Gene Idea AP Biology

  2. Gregor Mendel • Known as the “Father of Genetics” • Experimented with pea plants to develop principles of genetics • Peas exhibit complete dominance (he didn’t know this) • Easily manipulated

  3. Gregor Mendel Knew some “factor” caused organisms to have “characters” like their parents Now we call them genes and traits

  4. New Vocabulary, courtesy of Mendel Dominant: gene that is always expressed if present Recessive: gene that only expressed when 2 copies are inherited Homozygous/True-breeding/Pure: two of the same allele (ex: BB, bb) Heterozygous/Hybrid: two different alleles (ex: Bb) Phenotype: physical appearance of an organism (based on its genes) Genotype: genetic makeup of an organism (letters) P generation: parent generation (Mendel – true-breeding plants) F1 generation: first generation made from crossing P generation F2 generation: second generation made from crossing F1generation

  5. Mendel’s Experiments

  6. Monohybrid crosses: P generation • Mendel’s P generation plants were true-breeding (homozygous) • Gametes produced had one allele

  7. Monohybrid Crosses: F1 generation F1 generation were all heterozygous Some gametes P, some p. Provided evidence for Law of Segregation

  8. Monohybrid crosses: F2 generation • Recessive trait appeared again • Crossing two heterozygotes yields: • 3:1 phenotypic ratio (3 purple : 1 white) • 1:2:1 genotypic ratio (1 PP : 2 Pp : 1 pp)

  9. Mendel’s Laws • Law of Segregation • Two alleles of a gene separate during meiosis • Law of Dominance • An organism inherits two copies of each gene; if they are different, expression of the dominant gene masks the expression of the recessive gene • Law of Independent Assortment • Each allele pair segregates into gametes independently of other pairs (random)

  10. Inheritance Patterns • Complete dominance (as shown in Mendel’s pea plants) • Dominant is always expressed over recessive • Recessive only shows if both alleles are recessive • RR = purple; Rr = purple; rr = white

  11. Mendel was lucky – most inheritance follows other patterns… Let’s look at the exceptions to the norms

  12. Incomplete Dominance One allele is not completely dominant over another Heterozygous organisms display a blend between both phenotypes Example: snapdragons

  13. Codominance • Alleles do NOT blend • Both traits show in heterozygous organism • Both alleles expressed dominantly • Neither allele is recessive • Ex: roan cattle

  14. Multiple Alleles • Many genes have more than 2 alleles • Examples: • human blood type • fur color in rabbits

  15. Genes & Environment • Some traits are affected by the environment • Examples: • hydrangea flowers • Acidity of soil influences color • Himalayan rabbits • Fur color coincides with temp. • Paws, nose, ears, dark when cold, light when warm

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