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Mendelian Genetics

Mendelian Genetics. How pea plants and humans mix it up. Early Ideas About Heredity . People learned that it took two parents to make a baby Many early beliefs about how characteristics are transmitted. Blending theory. Early Research .

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Mendelian Genetics

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  1. Mendelian Genetics How pea plants and humans mix it up

  2. Early Ideas About Heredity • People learned that it took two parents to make a baby • Many early beliefs about how characteristics are transmitted. • Blending theory

  3. Early Research • Mendel was not the first to perform experiments with pea plants • British farmers (stem height) • Performed almost the same experiments • Obtained the same results • Over 200 years before • T A Knight (flower color) • Same experiments • Same results • 1790’s

  4. Gregor Mendel • Born in 1822 in the Chech republic • Joined an Augustinian order in 1843 • Flunked out of college, but made some great friends who showed him the value of good data • Using pea plants, found indirect but observable evidence of how parents transmit genes to offspring

  5. The Garden Pea Plant • Self-pollinating • True breeding (different alleles not normally introduced) • Can be experimentally cross-pollinated

  6. F1 Results of One Monohybrid Cross

  7. F2 Results of Monohybrid Cross

  8. Mendel’s Monohybrid Cross Results 5,474 round 1,850 wrinkled 6,022 yellow 2,001 green 882 inflated 299 wrinkled 428 green 152 yellow F2 plants showed dominant-to-recessive ratio that averaged 3:1 705 purple 224 white 651 long stem 207 at tip 787 tall 277 dwarf

  9. Mendel’s Theory of Segregation • An individual inherits a unit of information (allele) about a characteristic from each parent • During gamete formation, the alleles segregate from each other

  10. Now we know about Alleles, alternative forms of a gene

  11. Female gametes A a A AA Aa Male gametes a Aa aa Punnett Square of a Monohybrid Cross Dominant phenotype can arise 3 ways, recessive only one

  12. Test Cross • Individual that shows dominant phenotype is crossed with individual with recessive phenotype • Examining offspring allows you to determine the genotype of the dominant individual

  13. Homozygous recessive Homozygous recessive a a a a A A Aa Aa Aa Aa a A aa Aa aa Aa Punnett Squares of Test Crosses Two phenotypes All dominant phenotype

  14. A Dihybrid Cross - F1 Results purple flowers, tall white flowers, dwarf TRUE- BREEDING PARENTS: AABB x aabb GAMETES: AB AB ab ab AaBb F1 HYBRID OFFSPRING: All purple-flowered, tall

  15. ab ab aB AB AB Ab Ab aB 16 Allele Combinations in F2 1/4 1/4 1/4 1/4 1/4 1/16 1/16 1/16 1/16 AABB AABb AaBB AaBb 1/4 1/16 1/16 1/16 1/16 AABb AAbb AaBb Aabb 1/4 1/16 1/16 1/16 1/16 AaBB AaBb aaBB aaBb 1/4 1/16 1/16 1/16 1/16 AaBb Aabb aaBb aabb

  16. ab ab aB AB AB Ab Ab aB Explanation of Mendel’s Dihybrid Results If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible 1/4 1/4 1/4 1/4 1/4 1/16 1/16 1/16 1/16 AABB AABb AaBB AaBb 1/4 1/16 1/16 1/16 1/16 AABb AAbb AaBb Aabb 1/4 1/16 1/16 1/16 1/16 AaBB AaBb aaBB aaBb 1/4 1/16 1/16 1/16 1/16 AaBb Aabb aaBb aabb

  17. Phenotypic Ratios in F2 Four Phenotypes: • Tall, purple-flowered (9/16) • Tall, white-flowered (3/16) • Dwarf, purple-flowered (3/16) • Dwarf, white-flowered (1/16) AaBbX AaBb

  18. Independent Assortment • Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other trait • Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis

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