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Introduc)on to Gene)cs 02-­‐223 How to Analyze Your Own Genome Fall 2013 Overview • Primer on gene<cs – Cell, chromosomes, and DNA – Mendel’s experiment • Mendel’s three postulates • Mendel’s two laws of inheritance – Recombina<on Cells, Chromosomes, and DNA Diploid vs. Haploid Organisms • Diploid Organisms – 2 copies of each chromosome in a cell for most of its life cycle – e.g., Humans, fruit flies, mice • Haploid Organisms – 1 copy of each chromosome for most of its life cycle – e.g. Bacteria, yeasts and molds, some plants such as bryophytes (mosses) chromosomes chromosomes Diploid and Haploid Cells in Human Body Most cells in human body are diploid Egg/sperm cells are haploid Homologous Chromosomes in Diploid Cell • In each homologous pair of chromosomes, one member is derived from each parent. Gregor Mendel Greg Mendel (1822-­‐1884) St. Thomas’s Abbey, Brno, Czech Republic Green pea experiment Mendel’s Three Postulates of Inheritance I • Unit factors exist in pairs • Implies homologous chromosomes! Mendel’s Three Postulates of Inheritance II • In pair of unit factors for a single characteris<c in an individual: – one unit factor is dominant and the other is recessive D D: dominant d: recessive d Dominant vs Recessive • D: dominant d: recessive • Individuals with – DD: individuals show the trait associated with D – Dd: individuals show the trait associated with D – dd: individuals show the trait associated with d Mendel’s Three Postulates of Inheritance III • Paired unit factors segregate (separate) independently during gamete forma<on – Law of Segrega)on • Homologous chromosomes that can be separated to form haploid cells! Mendel’s First Law • Characters are controlled by pairs of genes which separate during the forma<on of the reproduc<ve cells (meiosis) Aa
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Mendel’s Experiment: Stage 1 Mendel’s Experiment: Stage 2 Mendel’s Experiment • Hypothesis: If Mendel’s first law was true, the ra<o of tall and dwarf peas should be 3:1 • Experimental valida<on: In Mendel’s green pea experiments, the ra<o was indeed 3:1 Mendel’s Pea Experiments Mendel’s Second Law • When two or more pairs of gene segregate simultaneously, they do so independently. A a B b Mendel’s Second Law • When two or more pairs of genes segregate simultaneously, they do so independently. A a; B b
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Mendel’s Second Law Green pea experiments again: Color G: yellow Shape W: Round GGWW
g: green w: wrinkled x
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16 possible genotypes "
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and 4 possible phenotypes ggww"
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GGWW GGWw GGwW GGww"
GgWW GgWw GgwW Ggww"
gGWW gGWw gGwW Ggww"
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Mendel’s Second Law “Excep)ons” to Mendel’s Second Law Morgan’s frui0ly data (1909): 2,839 flies Eye color A: red a: purple Wing length B: normal b: ves<gial AABB
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“Excep)ons” to Mendel’s Second Law • When two or more pairs of gene segregate simultaneously, they do so independently. D d W w G g Morgan’s explana)on A
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F2:"
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Crossover has taken place"
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Recombina)on • Parental types: AaBb, aabb • Recombinants: Aabb, aaBb – The propor<on of recombinants between the two genes (or characters) is called the recombina/on frac/on between these two genes. • Recombina/on frac/on It is usually denoted by r or θ. For Morgan’s traits: r = (151 + 154)/2839 = 0.107 If r < 1/2: two genes are said to be linked. If r = 1/2: independent segrega<on (Mendel’s second law). Now we move on to (small) pedigrees. How Can We Iden)fy the Unknown Disease Locus? • Idea: Given a map of gene<c markers, let’s look for the markers that are linked to the unknown disease locus (i.e. linkage between the disease locus and the marker locus) Disease Locus Marker near the disease locus (r<<0.5) Markers far from the disease locus (r=0.5) Summary • Mendel’s two laws of inheritance • Morgan’s experiment – Excep<ons to Mendel’s law – Recombina<on: two genes on the same chromosome can be linked and inherited together to offsprings.