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Lecture 4

Lecture 4 - Heredity and Evolution - October 2.docx

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University of Toronto St. George
Keriann Mc Googan

October 2, 2012. Lecture 4 – Heredity and Evolution This Class Overview  Genetic principles – Gregor Mendel  Mendelian Inheritance in humans  Non-Mendelian Patterns of Inheritance  Genetic and environmental factors  Modern evolutionary theory  Variation  Genetics and natural selection  Natural selection and behaviour Gregor Mendel  1822 – 1884  Silesian monk  University of Vienna, natural sciences  Experiments in monastery gardens o 1856 – 1863: Garden pea plants Some Definitions  Alleles: varieties of genes, corresponding pairs of genes located at specific locations on the chromosomes  Genotype: combination of genes or alleles an individual carries, genetic makeup of an individual  Phenotype: observable characteristics of the organism  Homozygous: individuals with the two copies of the same allele  Heterozygous: individuals with copies of two different alleles Mendel’s Experiments  7 different traits, 2 different forms o E.g. height = tall vs. short o Colour = green vs. yellow  Cultivated a population of true bred plants = parent plants  Cross parent plants that produce only tall plants with those that produce only short plants  F0generation = founding population (parent plants)  F1generation = first generation, offspring of founding population  F2generation = offspring of1F population  Blending inheritance: F generation should be intermediate in height 1  BUT F1generation all tall  Produce F2generation o ¾ of offspring tall, ¼ short  So: different expressions of traits controlled by “particles” (aka genes), offspring inherits one from each parent  1: Principle of Segregation (first principle of genetics) o Observed characteristics of organisms determined by two particles (genes) o One is inherited by the mother and one by the father  Expression that was absent i1 F generation was still there, but masked  Dominant Trait o Expressed trait  Recessive Trait o Unexpressed trait  2: Principle of Dominance and recessiveness o Recessive trait isn’t expressed in heterozygotes o Dominant traits prevent expression of recessive alleles  Different characteristics aren’t always inherited together o E.g., plant height and seed colour  3: Principle of independent assortment o Each of the two particles (genes) is equally likely to be transmitted when gametes form o Assort independently of each other Mendelian Inheritance in Humans  Mendelian trait = simple, discrete trait  18000 human characteristics are Mendelian traits  E.g., “hitchhiker’s thumb  Most Mendelian traits are biochemical Misconceptions  Confusion over dominance and recessiveness  Dominant alleles o Stronger or better (incorrect) o More common (incorrect)  Recessive alleles = “all or nothing” situation o Recessive alleles do still have influence o E.g., Tay-Sachs disease Human Patterns of Inheritance  Pedigree analysis: allows researchers to determine whether or not a trait is Mendelian  Six different modes of Mendelian inheritance o Autosomal dominant o Autosomal recessive o X-linked dominant o X-linked recessive o Y-linked o M…? Autosomal Dominant Traits  Governed by loci on dominant alleles  All affected members have at least one affected parent  Males and females equally affected o E.g. brachydactyly (short fingered hands) Autosomal Recessive Traits  Influenced by loci on autosomes  Recessive traits may appear to skip a generation if both parents carriers  Most affected individuals have unaffected parents  Frequency of affected offspring 50%  Males and females
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