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Beyond Mendelian Genetics

Beyond Mendelian Genetics. 14.3-14.4. Extending Mendelian genetics. Mendel worked with a simple system most traits are controlled by a single gene each gene has only 2 alleles, 1 of which is completely dominant to the other

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Beyond Mendelian Genetics

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  1. Beyond Mendelian Genetics 14.3-14.4

  2. Extending Mendelian genetics • Mendel worked with a simple system • most traits are controlled by a single gene • each gene has only 2 alleles, 1 of which is completely dominant to the other • The relationship between genotype & phenotype is rarely that simple

  3. Spectrum of Dominance • Alleles can show different degrees of dominance and recessiveness in relation to each other Incomplete Dominance Complete Dominance Codominance

  4. Complete Dominance Co Complete Incomplete • Mendel’s pea experiments = Complete Dominance • One allele completely dominant over the other • Phenotypes of heterozygote and dominant homozygote are indistinguishable • E.g. Pp and PP flowers both appear purple mutantallele producingmalfunctioningprotein wild typeallele producingfunctional protein homologouschromosomes

  5. Codominance Co Complete Incomplete • 2 alleles affect the phenotype in separate, distinguishable ways • 2 alleles produce two different, functional proteins • human ABO blood groups • http://www.youtube.com/watch?v=KXTF7WehgM8 Antigen (protein and carbohydrate = glycoprotein)

  6. ABO Blood Groups “I” stands for the enzyme that adds the antigen to the RBC • Multiple Alleles (3 alleles - IA, IB, i) • IA & IB alleles are co-dominant • Alleles code for enzyme that connects glycoprotein antigen on the surface of RBC • i allele is recessive to both IAIA or IAi IBIB or IBi IAIB ii

  7. Incomplete dominance Co Complete Incomplete • Heterozygotes have phenotype that is intermediate between 2 parental phenotypes • RR = red flowers • rr = white flowers • Rr = pink flowers • Heterozygotes produce less red pigment than the red homozygotes

  8. Incomplete Dominance = Blending? • If blending theory was correct, would you be able to produce a pure red or white carnation from a pink one? • No (this would take ∞ crosses!)

  9. Pleiotropy • So far we have assumed 1 gene = 1 phenotypic character • In reality most genes can have more than one effect on phenotype • E.g. Sickle Cell Anemia is mutation in one gene but produces multiple symptoms

  10. Extending Mendelian Genetics for Two or More Genes • So far we have considered the effects of the alleles of a single gene. • But, there are two situations where two or more genes are involved in determining a particular phenotype. • Epistasis • Polygenic Inheritance

  11. Epistasis • One gene alters the phenotypic expression of another gene (2 genes affecting 1 character) • coat color in mice = 2 separate genes • B,b:more pigment (black=B) or less (brown=b) • C,c:pigment (C) or no pigment (c) • cc = albino, no matter B allele • 9:3:3:1 becomes 9:3:4 B_C_ B_C_ bbC_ bbC_ _ _cc _ _cc

  12. Epistasis in Labrador Retrievers • 2 genes: (E,e) & (B,b) • pigment (E) or no pigment (e) • pigment concentration: black (B) to brown(b) E–B– E–bb eebb eeB–

  13. Polygenic Inheritance • Phenotypes determined by the additive effects of 2 or more genes on a single character • phenotypes on a continuum • human traits • skin colour • eye colour • height • weight • intelligence

  14. Any Questions? • Genetics Practice 2: Beyond the Basics • Inheritance of Blood Types Worksheet • Textbook Questions: • Pg. 146 #1,3,4,5,6,12 • Pg. 180 #4,9

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