Chapter 5_post

Genetics: A Conceptual Approach

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Chapter 5 Extensions and Modification Of Basic Principles Dr. Ed Otto George Mason University
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Key objectives Understand a number of extensions and modifications to Mendel’s basic principles: - different types of dominance - penetrance & expressivity - lethal alleles - multiple (>2) alleles - interactions between different genes - sex influences on heredity - continuous characteristics
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Dominance: Interactions Between Alleles
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Dominance One of Mendel’s importance contributions to genetics was the concept of dominance– the idea that an individual organism possesses two different alleles for a characteristic, but the trait encoded by only one of the alleles is expressed in the heterozygote When biologists began to apply Mendel’s principles to organisms other than peas, it became apparent that many characteristics do not exhibit this type of dominance
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3 Types of Dominance 1.Complete Dominance 1.Incomplete Dominance 1. Codominance Phenotype of heterozygote : - is the same as the phenotype of the homozygous dominant - is intermediate to the phenotype of the homozygous dominant and homozygous recessive - includes the phenotypes of both homozygotes (heterozygote simultaneously expresses the phenotype of both homozygotes)
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Examples 1.Complete dominance 1. Incomplete dominance 1. Codominance Mendel’s peas : RR = round rr = wrinkled Rr = round ( like homozygous dominan Eggplant : PP = purple pp = white Pp = violet ( intermediate ) MN Blood Types : L M L M = M blood type- expresses M antigen on red blood cells L N L N = N blood type- expressed N antigen on red blood cells L M L N = MN blood type- expressed M & N antigens on red blood cells ( simultaneously expresses both )
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Observing Phenotypes at Different Levels Phenotypes can frequently be observed at different levels, including anatomical, physiological or molecular Example: cystic fibrosis (CF) gene CF allele encodes a functional protein that transports chloride ions in and out of the cell cf allele encodes a dysfunctional protein; chloride ions build up inside cell
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Example: Cystic Fibrosis CF CF cf cf CF cf only functional protein produced only dysfunctional protein produced functional & dysfunctional proteins produced ( codominance ) person unaffected by disease person affected (suffers from cystic fibrosis: thick mucus, clogged airways, frequent life-threatening respiratory infections) person is unaffected ( complete dominance ), because there is enough functional protein produced by the single CF allele to prevent symptoms Genotype: Phenotype at Molecular Level: at Physiological Level: Type of dominance exhibited by a characteristic depends on the level of phenotype examined
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Important to remember that dominance affects the phenotype that genes produce, but NOT the way in which the genes are inherited Rr x Rr Pp x Pp gametes: ½ R ½ r ½ R ½ r ½ P ½ p ½ P ½ p fertilization: ½ R ½ r ½ P ½ p ½ R ¼ RR ¼ Rr ½ P ¼ PP ¼ Pp ½ r ¼ Rr ¼ rr ½ p ¼ Pp ¼ pp
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This note was uploaded on 05/06/2010 for the course BIO 311 taught by Professor Otto during the Spring '10 term at George Mason.

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Chapter 5_post - Chapter 5 Extensions and Modification Of...

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