Chapter 2 - Genetics

Chapter 2 - Genetics - Genetics The importance of Mendel...

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Genetics The importance of Mendel - Before Mendel’s ideas about heredity: o It was believed that traits are transmitted directly from parent to offspring o Dominant model of the time: inheritance is a matter of blending traits from the parents - Important ideas from Mendel: o Particulate inheritance o Quantitative approach o Segregation o Independent Assortment Nomenclature - Gene: information necessary to specify a trait (very simplistic, not always 1:1 ratio between a gene and a trait) - Chromosomes: how genetic information is packaged, contain many genes - Locus: location of gene on chromosome - Allele: different forms of a gene - Genetic marker: known location on a chromosome - Genotype: alleles carried by individual - Phenotype: appearance of individual True-breeding variants - Monohybrid cross: a cross to study only 2 variations of a single trait
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- Mendel produced true-breeding pea strains for 7 different traits o True-breeding: offspring look like parents - Each trait had 2 alternate forms (variations) - Mendel cross-fertilized the two true-breeding strains for each trait Monohybrid cross-1,2,3 (for drawings) and F1 generation - Cross 1: o Use some of Mendel’s original traits Seed shape: round and wrinkled o Cross true-breeding round to true-breeding wrinkled -> all round (F1 generation) - F1 generation: o F1 generation (1 st filial generation): offspring produced by crossing 2 true-breeding strains o For every trait Mendel studied, all F1 plants resembled only 1 parent o No plants with characteristics intermediate between the 2 parents were produced (no blending) - Monohybrid Cross (continue) o P0 X P0 = F1 X F1 = F2 o Wrinkled reappears in second generation o Count lots of seeds, find: ¾ round, ¼ wrinkled Phenotypic ratio = 3 round: 1 wrinkled o How to explain these results? Dominant and recessive
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- Alternate versions of genes account for different traits (different alleles for different traits) - Organisms inherit 2 alleles, one from each parent - If the alleles differ: o The dominant allele determines the phenotype o The trait we see in the F1 is dominant o The trait that disappeared is called recessive (somehow they’re still there thought not expressed in phenotype) - Rethink cross in terms of genotypes & gametes o Symbolize round allele with R o Symbolize wrinkled allele with r RRxrr - Gametes: RR Parent: R rr Parent: r - F1 Rr - F1xF1 RrxRr - Gametes: o Rr Parent: R, r Rr Parent: R, r - F2 1RR 2 Rr 1rr - Phenotypic Ratio = 3 Round: 1 wrinkled Principle of segregation - Crosses illustrate principle of segregation
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o Alleles remain distinct o Alleles for a gene segregate during gamete formation o 1 alleles in each gamete o Gametes can inherit either allele from a parent, but not from both - Homologues disjoin during Anaphase I o So alleles on homologues must as well Genotype and phenotype - Topic we will explore from different perspectives - Genotype determines phenotype: o RR dominant phenotype (homozygous) o Rr Dominant phenotype (heterozygous) o rr recessive phenotype (homozygous) - different genotypes can have same phenotypes Analyzing crosses (Punnet square) [figure 12.7]
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