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MENDELIAN GENETICS OBJECTIVES • Understand Mendel’s principles governing genetics • Understand meaning of relevant vocabulary • Predict results of mono/dihybrid crosses using Punnett square • Know exceptions to rules GENETICS Gene (Character): TERMINOLOGY Locus (loci= pl.): Brown Blue Allele (Trait): Eyes Eyes Black Genotype: Red Hair Hair Phenotype: GENETICS • Genetics: • Heredity: MENDEL • Gregor Mendel – Interested in basic patterns governing the transmission of traits from parent to offspring – Mendel’s work precedes an understanding of meiosis/genes/chromosomes HISTORICAL HYPOTHESES REGARDING INHERITANCE Blending Inheritance Hypothesis: – Offspring represent blend of characteristics from two parents Inheritance of Acquired Characteristics Hypothesis – Traits in parents are modified, through use, and passed on to offspring in modified form MENDEL • Mendel performed crosses between pure bred garden peas • Began studying inheritance of ONE trait • Later, multiple traits studied together MENDEL’S CROSSES • Mendel's plants had two observable “forms” (alleles) of each “character” (gene) – Green or Yellow seeds • Round or Wrinkled seeds Trait Forms of Trait (Phenotype) or Seed shape Round Wrinkled Round Wrinkled or Seed color Yellow Green Yellow Green Pod shape or Inflated Constructed Smooth Constricted or Pod color Green Green Yellow Yellow Trait Forms of Trait (Phenotype) Flower or color Purple White Purple White Flower and or pod position Axial (on stem) Axial (on stem) Terminal (at tip) Terminal (attip) Stem length or Dwarf Tall Tall Dwarf MENDEL • Looked at traits exhibited by progeny of crosses – He studied phenotype – Found observable evidence of how parents transmit traits to offspring MENDEL’S PEA PLANTS • Pisum sativum – Can self fertilize Male parts = anthers –Pollen grain contain sperm SELF FERTILIZATION Female parts = pistil –Ovule = eggs Stigma (receives pollen) Anthers (produce pollen grains, which contain male gametes) Ovules (produce female gametes) MENDEL’S PEA PLANTS Controlled Mating: – Cut off reproductive organ – Cross fertilize plants in a controlled fashion – Plant that contributes pollen – Plant that receives the pollen CROSS-POLLINATION 1. Remove anthers from one plant. 2. Collect pollen from a different plant. 3. Transfer pollen to stigma of the 1st plant (plant without anthers) MENDEL’S CROSSES Round Parent x Wrinkled Parent All Round Offspring (F1) (Wrinkled trait disappeared!) MENDEL’S CROSSES Next, Mendel crossed F1 (bro-sis mating): Round x Round (F1) Get both types of offspring (F2) Wrinkled trait re-emerges in F2 MENDEL’S CROSSES Mendel Determines (Phenotypic) Ratio: F2: 5474 Round : 1850 Wrinkled » Nearly 3 : 1 – Similar pattern observed in other 6 traits MENDEL’S CONCLUSIONS • There are alternative versions of genes • For each inherited character, an organism inherits TWO alleles • Inherit same two alleles • Inherit two different alleles • If the two alleles differ, one allele determines the organism’s appearance • Each gamete carries ONE allele for ea. character MENDEL’S SYMBOLS • Mendel assigned symbols to hereditary characters • Letters represent versions of a gene (alleles): – Ex: If Letter “r” represents a gene then: – R symbolizes dominant allele – r symbolizes recessive allele PUNNETT SQUARE • R.C. Punnett – Leading geneticist in early 1900s – Invented technique called the “Punnett Square” – Place gamete possibilities for 2 parents along axes – Internal boxes represent union of gametes – Used to predict genotype and phenotype of potential zygote MENDEL’S CROSS OF PURE LINE PEA PLANTS PARENT: R R R is round PARENT: r r r is wrinkled ** hint: gametes along top, “whole people” on inside MENDEL’S CROSS OF F1 PLANTS Crossing the F1s: Rr x Rr R = round r = wrinkled • Phenotypic Ratio: • Genotypic Ratio POSSIBLE GENOTYPES Three possible genotypes for each gene in a diploid cell: • Homozygous: • Homozygous Dominant: • Homozygous Recessive: • Heterozygous: PROBABILITY THEORY In Diploid Organisms, • Each allele has 50% chance of being found in a particular gamete • To calculate likelihood of two alleles combining (i.e. fertilization): MENDEL’S CROSSES • Dihybrid Crosses: – Mendel used pea plants to follow inheritance of TWO traits • Another pattern began to emerge… MENDEL’S SYMBOLS • Seed Shape: • R is dominant allele (Round) • r is recessive allele (Wrinkled) • Seed Color: • Y is dominant allele (Yellow) • y is recessive allele (Green) F2 OFFSPRING FROM DIHYBRID CROSS F2 generation phenotype Mendel’s 315 101 108 32 = 556 Numbers Fraction of /16 /16 /16 /16 = 1 progeny Note: New mixtures of traits are observed in F2!! PUNNETT SQUARE FOR TWO TRAITS Do genes segregate into gametes together or independently? – If together, we end up with only 2 potential phenotypes in the F2 generation PUNNETT SQUARE FOR TWO TRAITS If genes segregate independently of each other, we end up with FOUR different phenotypes – Some combos of traits in F2 are unique; differ from parental generation MENDEL’S LAW OF INDEPENDENT ASSORTMENT • Law of Independent Assortment: INDEPENDENT ASSORTMENT During meiosis I, homologs can line up two different ways before separating. Brown eyes Blue eyes OR Black hair Red hair Brown eyes Blue eyes Brown eyes Blue eyes Black hair Red hair Red hair Black hair THE TEST CROSS • To determine genotype of an individual Homozygous THE TEST CROSS recessive parent YOUR rryy HOMEWORK! ? All ry 1/4 1/4 1/4 1/4 MENDEL Why Mendel Was Successful: • Unbiased • Selected good model organism • Used pure breeds as parents • Large sample size • Quantitative analysis • Controlled experiments • Studied obvious traits CONCLUSIONS: MENDELIAN GENETICS • Mendel studied traits influenced by a single gene (two alleles) • Traits exhibited complete dominance • Autosomal inheritance
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