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Biology 12

Biology 12. Unit 3: Heredity Mendelian Genetics. Genes and Heredity. Mendel’s Experiments Alleles Punnett Square Monohybrid & Dihybrid Crosses. Mendelian Genetics. Traits are controlled by genes Law of Dominance: dominant and recessive Law of Segregation: genes separate during meiosis

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Biology 12

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  1. Biology 12 Unit 3: Heredity Mendelian Genetics

  2. Genes and Heredity • Mendel’s Experiments • Alleles • Punnett Square • Monohybrid & Dihybrid Crosses

  3. Mendelian Genetics • Traits are controlled by genes • Law of Dominance: dominant and recessive • Law of Segregation: genes separate during meiosis • Law of Independent Assortment: genes for one trait will separate independently of how other traits separate

  4. More Mendelian • Mendel worked with pea plants • True breeding tall plants are homozygous for the tall (T) allele • True breeding dwarf plants are homozygous for the dwarf (t) allele • When the parental generation (P1) was made up of true breeding tall and true breeding dwarf plants all offspring were tall

  5. P1 phenotype: Tall X Dwarf genotype: TT X tt gametes: T t F1 phenotype: Tall X Tall genotype: Tt X Tt gametes: T t T t F2 phenotype: 3 Tall : 1 dwarf genotype: 1 TT : 2 Tt : 1 tt

  6. T T T t t T Tt Tt TT Tt t t Tt Tt Tt tt Punnett Square • This F1 generation of tall plants were all heterozygous: one tall allele and one dwarf allele • When F1 individuals were crossed Mendel found a 3:1 ratio of tall to short plants

  7. f C B E A D f B d e A C c D A f B e Cc BB Dd ee AA ff DD ff BB Ee AA Cc b c f e d A Dd ff Ee Cc AA Bb ff AA Cc Bb ee dd Possible Crosses

  8. Monohybrid Cross

  9. t t T Tt Tt ? ?t ?t Test Cross • When an individual showing the dominant phenotype is crossed with a homozygous recessive individual to determine the genotype If dwarf individuals are observed in the offspring, then the unknown genotype is heterozygous

  10. Dihybrid Crosses • Mendel looked at two traits at a time to determine the Law of Independent assortment • True breeding Yellow Round Seed cross with true breeding green wrinkled seed

  11. Let Y be yellow, y is green Let R be round, r is wrinkled P1: genotype YYRR X yyrr gametes YR yr F1: genotype YyRr X YyRr gametes YR Yr yR yr F2: YR Yr yR yr YR Yr yR yr

  12. YR Yr yR yr YR YYRR YYRr YyRR YyRr Yr YYRr YYrr YyRr Yyrr yR YyRR YyRr yyRR yyRr yr Yyrr YyRr yyRr yyrr Dihybrid Crosses

  13. Mendel’s First & Second LawThe Law of Segregation &The Law of Independent Assortment

  14. Hr Hw E2 E3 CR CW Hr HrHw E1 E1E3 CR CRCW HrHr E1E2 CRCR Hw HrHw HwHw E4 E2E4 E3E4 CW CRCW CWCW Non-Mendelian Inheritance • Multiple Alleles • Example: Eye colour in Drosophelia • Wild Type > Apricot > Honey > White • Incomplete Dominance • Example: Snap dragons, red X white => pink • Co-Dominance • Example: Shorthorn cattle, red X white => roan

  15. Polygenic Inheritance • Gene interaction: Example Chicken Comb • Rose (R-pp) Single (rrpp) • Walnut (R-P-) Pea (rrP-)

  16. Pleiotropic genes • One gene with multiple effects • Example: Sickle-cell anemia • Lethal Genes • When a specific genotype is not viable • eg. if AA is lethal, a heterozygous cross would give a 2:1 ratio (2 Aa : 1 aa – the AA is dead)

  17. Non-Mendelian Genetics Clip 2 Clip 1

  18. Linkage • Mendel demonstrated his Law of Independent Assortment using simple traits. • Some traits do not follow this Law. • When genes are on the same chromosome they do not separate independently

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