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LS4 - Lecture 2 HW chapter 2 15-37 Lecture 3 HW chapter 3 2...

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Lecture 2 HW: – chapter 2 15-37 Lecture 3 HW: – chapter 3: 2 - 7, 8 a&b, 9 - 11, 13 - 15, 17 - 19
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Variations of Mendelian Phenotypic Ratios • Alleles can interact with each other in complex ways - incomplete dominance - codominance • Pleiotropic affect • Many traits are controlled by more than one gene - epistasis - redundancy - complementation • Penetrance and Expressivity • Interactions of genotypes with the environment
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Interactions between the alleles of a single gene
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• Mendel established… -Genes are in pairs and can exist in alternate forms ( alleles ) -These alleles can be dominant or recessive • This suggested a simple functional dichotomy between alleles--as if one allele did nothing and the other did everything to determine the phenotype. X Purple White Purple F1 Variations of Mendelian phenotypic ratios
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X purple white purple F1 • But nature is usually more complicated than this. • Mendel was successful because he chose simple examples to start with. Colorless Precursor purple pigment purple pigment forming enzyme P = dominant allele makes a normal enzyme p = recessive allele makes a defective one So far we have looked at traits that are clearly dominant or recessive.
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Incomplete dominance : The phenotype of the heterozygote is an intermediate between homozygous parents. Exception to Simple Dominance: Incomplete Dominance Segregation of both genotype and phenotype is 1:2:1. Alleles contribute different amounts of functional protein; total amount determines phenotype.
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P Generation Red RR White rr Gametes R r F 1 Generation Pink Rr Gametes 1 / 2 R 1 / 2 r 1 / 2 R 1 / 2 R Eggs Sperm 1 / 2 r 1 / 2 r F 2 Generation Red RR Pink rR Pink Rr White rr Incomplete Dominance in Plants • Example: snapdragons • In incomplete dominance F 1 hybrids have an appearance in between the phenotypes of the two parents. • The phenotypic ratio and genotypic ratio are the same (1 RR, red:2 Rr, pink:1 rr, white) -Mendel’s laws are still valid at the genotypic level .
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Precursor Red pigment Red pigment forming enzyme A = Dominant allele makes a normal enzyme
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