Download - The Chromosomal Basis for Inheritance
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The Chromosomal Basis for Inheritance
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Thomas Hunt Morgan
Early 1900sColumbia University (New York)Studied genetics of Drosophila melangaster (the common fruit fly)
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Why Drosophila?Only four pairs of chromosomesA single mating produces hundreds of offspringA new generation can be produced every 2 weeks
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Morgan's studies revealed...Genes are located on specific chromosomes at specific loci (locations).There are many genes on a single chromosome.Genes can be linked to various degrees (i.e. they can be inherited together if they are located on the same chromosome.)
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Sex-linkageMorgan discovered that Drosophila eye color was linked to the X chromosome, supporting the chromosome theory of inheritance.X linkage and Y linkage exist
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PracticeA white-eyed female fruit fly is mated with a wild-type male. What are the expected phenotype and genotype ratios?
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X InactivationBarr bodiesInactivation is varied, leading to a mosaic of traitse.g. Tortoiseshell cats, sweat glands in human females
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Note that various systems of sex determination exist.X-YX-OZ-WHaplo-diploid
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Morgan's research supports Mendel's Laws
Homologous chromosomes account for Mendel's Law of Segregation.Non-homologous chromosomes account for Mendel's Law of Independent Assortment
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Linked GenesGenes located near each other on the same chromosome tend to be inherited together in genetic crosses.Refers to two or more genes on a single chromosome. (This is not the same as sex-linkage.)
Normal vs vestigial wings
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PracticeLet b+ = gray body and b = black body. Let vg+ = normal wings and vg = vestigial wingsWhat is the expected phenotype ratios if a fly heterozygous for both traits is crossed with one that is homozygous recessive for both traits?
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Genetic RecombinationParental typesNon-parental types = recombinant types = recombinantsIf chromosomes independently assort, 50% recombination frequency is expected.Linkage is suspected when recombination is below 50%.Due to crossing-over.
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Linkage/genetic mapsBased on the idea that the farther apart two genes lie on a chromosome, the more likely a cross over event will occur between them.1 map unit = 1% recombination frequency
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For example...Three genes (b, cn, and vg) are found on a single chromosome. Recombination frequencies are b-cn 9%, cn-vg 9.5%, and b-vg 17%.
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PracticeGenes A, B, and C are located on the same chromosome. Testcrosses show that the recombination frequency between A and B is 28%, B and C is 5%, and A and C is 12%. What is the linear order of these genes?
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PracticeDetermine the sequence of genes along a chromosome based on the following recombination frequencies: A-C 28%, A-B 8%, A-D 25%, B-C 20%, B-D 33%.