Lec13Notes - Genetic basis of heredity 2 Principles of...

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Genetic basis of heredity 2 Principles of Biology Lecture 13 Prof. David Fitch
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Genetics terms = concepts Phenotype Character Trait Genotype Homologous chromosomes Sister chromatids Locus Gene Gene copy Allele Dominant / Recessive Incomplete dominance / Codominance
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Last time: Mendel's model for a single locus : 4 principles 1. Different alleles of a gene account for the difference between traits of a character 2. 2 gene copies are inherited for each character (diploidy) derives from the binomial distribution of genotypes 3. If heterozygous, the dominant allele determines phenotype (not the recessive one) 4. " Law of Segregation " maternal and paternal gene copies segregate and end up in different gametes thus, each gamete has only one gene copy for each locus gene copies will be different alleles if genotype is heterozygous Random fertilization produces 1:2:1 of zygote genotypes: ( p + q ) ( p + q ) = p 2 + 2 pq + q 2 gene copy 1 gene copy 2 genotype1 genotype 2 genotype 3
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Punnett square From the hybrid ( Pp ), because of the Law of Segregation , predict 50% of sperm have P , 50% of sperm have p Punnett square can be used to predict ratios of genotypes and phenotypes in F 2 Phenotype: P Generation Genotype: Gametes F 1 Generation Phenotype: Genotype: Gametes: F 2 Generation Purple flowers Pp P p 1 2 1 2 P p F 1 sperm F 1 eggs PP Pp Pp pp P p 3 : 1 Purple flowers PP White flowers pp P p
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Two loci P Generation F 1 Generation YYRR Gametes YR yr yyrr YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Sperm Eggs YR Yr yr YR YR yr Eggs YYRR YyRr YyRr yyrr yR yr Phenotypic ratio 3:1 F 2 Generation (predicted offspring) YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr Phenotypic ratio 9:3:3:1 YR Yr yR yr Sperm 1 2 1 4 1 4 1 4 1 4 1 4 3 4 1 2 1 2 1 2 1 4 9 16 3 16 3 16 1 4 1 4 1 4 Law of In- dependent Assortment alleles at different loci assort independently into gametes dihybrid 1/16
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Different kinds of interactions between alleles Incomplete dominance – heterozygotes are intermediate in phenotype Codominance – different phenotypes expressed together (e.g. ABO, MN) "Overdominance" – heterozygote advantage (e.g. sickle-cell anemia) Epistasis – alleles at one locus alter expression of alleles at another locus (e.g. albinism) Polygenic inheritance – additive effects of alleles at multiple loci on the same phenotype
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Epistasis Interaction between loci : one gene affects the expression of another – e.g., C is needed for color deposit; cc is albino, regardless of allele at B locus Sperm BC bC Bc bc BbCc BBCc BbCC BBCC BbCC bbCC BbCc bbCc Bbcc BBcc BbCc BBCc BbCc bbCc Bbcc bbcc BC bC Bc bc BbCc BbCc 1 4 1 4 1 4 1 4 1 4 1 4 1 4 1 4 9 16 3 16 4 16
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Polygenic inheritance Quantitative traits can be determined by several genes – e.g. additive interactions
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Lec13Notes - Genetic basis of heredity 2 Principles of...

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