2. Beyond Mendelian Genetics 14.3-14.4

3. 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

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

5. Complete Dominance 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 homologous chromosomes wild type allele producing functional protein mutant allele producing malfunctioning protein Complete Incomplete Co

6. Codominance 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 http://www.youtube.com/watch?v=KXTF7WehgM8 Complete Incomplete Co Antigen (protein and carbohydrate = glycoprotein)

7. ABO Blood Groups Multiple Alleles (3 alleles - I A, I B, i) – I A & I B alleles are co- dominant Alleles code for enzyme that connects glycoprotein antigen on the surface of RBC – i allele is recessive to both “I” stands for the enzyme that adds the antigen to the RBC I A I A or I A i I B I B or I B i IAIBIAIB ii

8. Incomplete dominance 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 Complete Incomplete Co

9. 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!)

10. 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

11. 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

12. Epistasis B_C_ bbC_ _ _cc 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

13. 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 eeB–eebb

14. 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

15. 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