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Human Biology
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Mendelian Genetics for Single Gene Traits Lecture overview 1) Mendel’s experiments: monohybrid and dihybrid cross, laws of segregation and independent assortment, dominance and recessiveness, test cross 2) Basic probability rules: product rule, sum rule [Genetics Before Mendel: Problems with Inheritance] organisms tend to resemble their relatives, BUT: • Why do siblings look different from each other? • Why do some characteristics disappear and then reappear over several generations? • How do new characteristics appear? • is inheritance determined by both parents (biparental) or only 1 (uniparental)? • can an organism pass on characteristics acquired over its lifetime? [Mendel: What did he do differently?] -examined traits individually, rather than simply considering whole organism -controlled experiments with careful recording and counting -chose a good ‘model’ organism (peas) [What Makes a Good Genetic Model Organism?] -rapid generation time (can go across multiple generations in short time) -easy to grow/breed in large number -can examine large numbers of offspring (important when we get quantitative values) -can self-fertilize FOR MENDEL: • possess a number of ‘simple’, discrete variable traits that can be clearly recorded Discrete (ex. either green or red) ↔Continuous (complex inheritance) *7 discrete traits in peas [Usefulness of Self-Fertilization] -quick and easy to make “pure-breeding/inbred lines” -they are genetically homogeneous, showing same characteristic over generations [Dominance] One of the two traits in an antagonist pair (flower: purple/white) was “dominant" and would always be manifest in the hybrid (in F1(the next generation)only purple flowers) (+)The dominants in peas: long stems, axial flowers, purple flowers, inflated pot, green pot, yellow pea, round pea • Is inheritance determined by both parents (biparental) or only 1 (uniparental)? └immaterial(doesn’t really matter)! The one with dominant trait will determine! This was revealed by “Reciprocal Crosses (ex: purple(mom)white(dad) and another cross of purple(dad)white(mom) ). This revealed that purple is dominant trait, and the inheritance determination is “Independent of the parent.” When we cross peas, we are considering only one trait/ characteristic. [Mendel’s Model to Explain the Results] One individual has 2 alleles. Uppercase for Dominant and lowercase of recessive alleles.
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