Unit 2 Review Slides.pdf - Historically speaking the...

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2/11/18 1 Problems with the Theory of Natural Selection in 1859 1. Darwin could not explain the mechanism by which parents passed on traits to their offspring. 2. Darwin could not explain how variation was maintained (he believed in blending inheritance). 3. Darwin could not explain how new variation arose. Historically speaking, the predominant belief was… X Blending Gregor Mendel and Pea Plants Parent Generation (purebred) F 1 F 2 To explain this pattern, Mendel introduced the concept of Dominant and Recessive Traits Dominant traits ( ) are always expressed. Recessive traits ( y ) are masked whenever they are paired with a dominant allele. Mendelian Principle #1 The Principle of Segregation Genes ( alleles ) occur in pairs, but only one copy is transmitted through a gamete to the offspring. YY yy P generation Y y y Y Gametes Y y Zygote
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2/11/18 2 Mendelian Principles The Principle of Segregation § Factors (now called genes) occur in pairs, but only one copy is transmitted through a gamete to the offspring. § Each parent contributes one copy of the gene. § Different versions of the gene are now called alleles. Genotype and Phenotype Phenotype = external appearance ( yellow or green ). Genotype = what is in an organism’s genes. 3 possible genotypes: YY homozygous dominant Y y heterozygous (dominant expressed) yy homozygous recessive Y y Y y Y y Y y Parental purebred gametes Yellow Yellow Yellow Yellow 4:0 phenotypic ratio 4:0 genotypic ratio How can we determine the genotype of F 1 ? Mendel went another step: Two traits: S/s = smooth/wrinkled Y/y = yellow/green • When a gamete is transmitted, it only takes one form of the gene (i.e. allele) with it. • In this case, there are 2 alleles for color and 2 alleles for shape . There is no reason why smooth (S) must always pair with yellow (Y) because it has equal chance to pair with green (y). Thus, there are four possibilities... Therefore… • Genes controlling different traits are inherited independently of one another. • The distribution of one pair of alleles into gametes does not influence the distribution of another pair .
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2/11/18 3 Mendelian Principle #2 The Principle of Independent Assortment § The distribution of one pair of alleles into gametes does not influence the distribution of another pair.
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