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Chapter 13 Raven & Johnson Chapter 15 Campbell Chromosomes & Inheritance
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Mendel & Genes 1902 – Chromosomal Theory of Inheritance Genes have specific loci on chromosomes – chromosomes divide and move
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Thomas Hunt Morgan’s Thomas Hunt Morgan’s work Drosophila melanogaster (fruit fly) Normal phenotype – wild type – red eyes Mutant phenotype – white eyes Discovered through mating that white eyes was sex linked Gene located on sex chromosome
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Thomas Hunt Morgan’s Work
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Thomas Hunt Morgan’s work If on X chromosome, females need two copies to express recessive gene Males need only one (they are XY) Females with one – carrier (can pass on)
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Thomas Hunt Morgan’s work Linkage – genes close to each other tend to be inherited together
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Genetic Recombination Production of offspring with new combinations of traits inherited from two parentsProduction of offspring with new combinations of traits inherited from two parents If unlinked, there is a 50% frequency of recombinationIf unlinked, there is a 50% frequency of recombination
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Completely linked have no recombinants – 0% frequency If normally linked, then crossing over could break the linkage and raise the frequency
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Genetic Mapping Alfred Sturtevant The farther apart two genes are on a chromosome, the more chances that crossing over can occur and the higher the recombinant frequency will be Can use frequency numbers to map the distance and order of genes – linkage map
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Genetic Mapping Testcross design to map the relative position of three fruit fly genes, body color (b), wing size (vg), and eye color (cn). The recombination frequency between cn and b is 9%. The r.f. between cn and vg is 9.5%. The r.f. between b and vg is 17%.
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Genetic Mapping The only possible arrangement eye color gene between the other two Map unit – distance between genes (1 m.u. = 1% recombination frequency) Distances are added
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Sex Chromosomes XX – female XY – male Determined by sperm (X or Y) Sex linked – usually on X & recessive Fathers pass to all daughters but no sons Mothers can pass to either
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Sex Chromosomes Ex - colorblindnesscolorblindness
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Sex-linked Disorders Dechenne muscular dystrophy Progressive muscle weakness Missing protein (dystrophin) Hemophilia Absence of protein for blood clotting
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X-Inactivation Females have two X chromosomes, but one inactivates during embryo development Condenses into Barr body So only one X functions in males AND females Occurs when –CH 3 groups are added to cytosine Gene on inactive X makes this happen A lot more to figure out
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Chromosome Errors Nondisjunction – homologous chromosomes or sister chromatids do not split evenly into gametes Result – some have more, some have less Abnormal chromosome # - aneuploidy Extra copy – trisomic 1 copy missing – monosomic Extra full sets – polyploidy (occurs in plants)
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Chromosome Errors Deletion – missing part Duplication – extra piece
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Chromosome Errors Inversion – reversal Translocation – out of order
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Resulting Problems Many result in spontaneous abortion Down syndrome Trisomy 21 Specific symptoms Correlation with maternal age Result of nondisjunction during meiosis I
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Resulting Problems XXY – Kleinfelter’s syndrome XO – Turner syndrome Deletion at #5 – cri du chat Translocations – some cancers Fragile X – abnormally thin section of DNA on X chromosome Symptoms are different if from sperm or egg (worse if from egg)
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FYI…Cytoplasmic genes Genes found on DNA of mitochondria and chloroplasts (not nucleus) do not follow all the rules
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