class 9 mutations 2 exceptions (1)

class 9 mutations 2 exceptions (1) - B BIO 360 Class 12...

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DNA structure: double helix. Two strings of nucleotides. changes to the sequence: mutations and their consequences Mutations (continued)  B BIO 360, October 31, 2012 Class 12 gain-of-function mutations exceptions to Mendel’s rules o incomplete dominance o co-dominance o lethal mutations o mitochondrial inheritance
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How mutations affect phenotype GOF : Gain Of Function mutations result in a functional protein that… …is made at the “wrong” place …is made at the “wrong” time …has a new activity Why? only very specific mutations will have this effect examples: specific amino acid changes promoter mutations Very few mutations are GOF. GOF mutations need not be beneficial
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How mutations affect phenotype Will GOF mutations be dominant or recessive? can you predict? depends on threshold! (but there are exceptions to these generalizations) for example, only a small amount of the altered protein is sufficient to produce the mutant phenotype Most GOF mutations are dominant
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Antennapedia in Drosophila Wild type Antennapedia gene is normally expressed only in the thorax; legs are made. GOF example #1–normal protein, wrong place
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Antennapedia in Drosophila A mutation causes the Antennapedia gene to be expressed in the thorax and also  in the head , where legs result instead of antennae! GOF example #1–normal protein, wrong place What phenotype would you predict for the heterozygote? What kind of mutation is this?
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What do you think a homozygous LOF Antennapedia (that is, LOF of the normal Antennapedia gene) mutant would look like? a. Normal fly b. Antennae on head and thorax c. Legs on head and thorax d. No legs on thorax and normal antennae on head e. No legs on thorax and no antennae on head
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age 3 15 lactase activity Homozygous wt can digest lactose cannot Homozygous mutant age 3 15 Gene promoter: ON OFF ON ON GOF example #2–normal protein, expanded time lactose tolerance in humans
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age 3 15 lactase activity Homozygous wt can digest lactose cannot which allele is dominant? what would be the phenotype of the heterozygote? Homozygous mutant age 3 15 Gene promoter: ON OFF ON ON mutant has gain of function expect lactose tolerance to be dominant GOF example #2–normal protein, expanded time
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Lactose Intolerance: Worldwide Distribution
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GOF or LOF Defined by comparison with the normal properties of the gene, not of the organism Is it LOF or GOF? Ask yourself what would happen if the gene were missing altogether
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…also called  dominant-negative  mutations Amount  of active  protein Aa “wt” phenotype “mutant”  phenotype why does Aa have so little activity?
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