Lecture IV - Hardy-Weinberg

Lecture IV - Hardy-Weinberg - Lecture IV Genetic Variation...

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Lecture IV – Genetic Variation, Hardy-Weinberg Equilibrium
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Terminology First, some basic genetic terminology: Gene : unit of heredity, codes for a protein Locus : position of a gene or a regulatory unit (or other significant DNA sequences) Allele : variation of a gene Homozygote : an individual with two copies of the same allele for a gene Heterozygote : an individual that has two different alleles at
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Terminology The simplest presentation of genotypes is using letters, or in diploid organisms, a pair of letters Upper case letters represent dominant alleles, lower case, recessive. AA, Aa, and aa are genotypes for gene A
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Terminology In genetic research, genes and alleles are often written using a letter to represent a gene with a mutation in superscript: vestigial winged fruit fly genotype: W vg W vg or vg / vg (shorthand) The wild-type allele is always +, so normal fly would be W + W + or + / + , and a heterozygote, W vg W + or vg / +
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Mathematical Genetics Ova, and sperm (pollen) even more so, are produced in very large quantities Thus, genotypes of progeny are the result of probabilistic events, rather than forgone conclusions So, to understand laws of genotypic inheritance, it is first important to understand some basic terms in probability and their use in genetics http://magickcanoe.com/frogs/frog-eggs-15-04-1-large.jpg
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Punnett Square A mathematical tool commonly used in genetics that allows the estimation of probabilities of inheritance of genotypes A A a A a A a a A a A a When breeding homozygous individuals with genotypes AA and aa , all progeny have same genotype - A a
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Punnett Square - B When breeding two heterozygous individuals of genotype A a , results are different A a A AA A a a A a aa The proportions are, in this case: AA : 1 A a : 2 aa : 1
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Multi-gene crosses The frequencies would be: AABB 1 (0.0625) AABb 2 (0.125) AAbb 1 (0.0625) AaBB 2 (0.125) AaBb 4 (0.25) Aabb 2 (0.125) aaBB 1 (0.0625) aaBb 2 (0.125) aabb 1 (0.0625) AB Ab aB ab AB AABB AABb AaBB AaBb Ab AABb AAbb AABb Aabb aB AaBB AaBb aaBB aaBb ab AaBb Aabb aaBb aabb Punnett squares can also be used for calculation of frequencies of genotypes when more than a single gene is being used in a cross http://www.bdjnews.com/confused.bmp/confused-full.jpg
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Mathematical Genetics - back The number for each genotype are the expected proportions of each genotype in case of a breeding event by that type of parental genotypes Another way of presenting that type of expected results would be using expected frequencies as a fraction of 1 (the total progeny). In this case: AA : 0.25 A a : 0.5 aa : 0.25
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Punnett Square – Assumptions The frequencies estimated by the Punnett square assume: No differences in survival of genotypes Large sample size
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Genotypic Frequencies Evolutionary processes do not take place at the level of the individual Evolution is the changes in the average phenotype of a population over time For changes to occur and be retained in a population, the frequencies of the genotypes have to change In other words, Evolution = changes in allele frequencies
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