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Genes & Heredity Mendelian Genetics Gregor Mendel (1822 – 1884) • Austrian monk • father of genetics • provided the basis for our understanding of heredity • in his time he had no knowledge of: 1. DNA 2. chromosomes 3. meiosis • experimented with garden peas Why study pea plants? 1. easy to grow 2. mature quickly 3. easy to pollinate 4. have contrasting traits • ex. height • seed colour • seed shape Mendel’s Experiments Experiment 1- Part A Monohybrid Cross: (one trait studied) height • crossed: P generation (parental) purebred tall F1 1st generation: x purebred short Predicted? All medium height Found? All tall Hypothesized? Short trait was lost Mendel’s Experiments Experiment 1- Part B • crossed: tall x tall F2 2nd generation: Predicted? All tall Found? 75% tall & 25% short Hypothesized? Short trait was not lost, just covered up by tall trait Mendel’s Laws Principle of Dominance: For contrasting traits : • a dominant trait can ‘mask’ the recessive trait Principle of Segregation: • Each offspring inherits two ‘factors’ (alleles) for each trait (one on each homologous chromosome) • these ‘factors’ (alleles) segregate during the formation of sex cells predicted • i.e. each gamete has only one allele meiosis Definitions gene: • section of DNA that codes for one characteristic • ex. height in pea plants allele: • variations of a gene • ex. tall vs. short • all alleles are represented by the same letter dominant allele: • codes for a trait that is expressed (visible) • represented by a capital letter ex. T (for tall) recessive allele: • codes for the trait that is not expressed (covered up) • represented by a lower case letter ex. t (for short) Definitions genotype: • the pair of alleles present in an organism • TT or Tt or tt homozygous genotype: • both alleles are the same TT homozygous dominant tt homozygous recessive heterozygous genotype: • both alleles are different Tt phenotype: • physical appearance of organism • determined by the combination of alleles (genotype) Definitions heredity: • the passing of traits from parents to offspring Mendelian Genetics Experiment 1: Part A P generation: purebred tall purebred short TT tt genotype: meiosis possible alleles: T T t t punnett square: • used to predict all possible genotypes from a cross alleles of 1 parent alleles tes of 2nd parent F1 T T t Tt Tt t Tt Tt phenotype: 100% tall genotype: 100% Tt (heterozygous) Mendelian Genetics Experiment 1: Part B F1 generation: hybrid tall x hybrid tall genotype: Tt Tt meiosis possible gametes: T t T t F2 T t T TT Tt t Tt t t phenotypes: 75% tall 25% short phenotypic ratio 3:1 dom:rec genotypes: 25% (TT) homozygous dominant 50% (Tt) heterozygous 25% (tt) homozygous recessive Mendelian Genetics Practice Problem: Predict the genotypes and phenotypes of the following cross: P generation Tt x tt phenotypes ? tall short F1 T t Genotypes: ? t Phenotypes: ? t Mendelian Genetics Practice Problem: Predict the genotypes and phenotypes of the following cross: T t x t t T F1 t Genotypes: 50 % T t 50 % t t t Tt tt Phenotypes: 50 % tall t Tt tt 50 % short Homework Handouts: 1. Genetic Problems ‘Monohybrid Crosses’ # 1-7 GENETICS PROBLEMS MONOHYBRID CROSSES 1. a) Gg 1. b) GG Gg gg GENETICS PROBLEMS MONOHYBRID CROSSES 2. a) b) c) 25 % will have blue eyes GENETICS PROBLEMS MONOHYBRID CROSSES 3. a) Yy yy b) YY Yy yy GENETICS PROBLEMS MONOHYBRID CROSSES 4. 5. GENETICS PROBLEMS MONOHYBRID CROSSES 6. 7.