extMendGenetics

extMendGenetics - Extensions to Mendel Complexities in...

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1 Extensions to Mendel Complexities in Relating Genotype to Phenotype
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2 Outline of extensions to Mendel’s analysis Single-gene inheritance In which pairs of alleles show deviations from complete dominance and recessiveness In which different forms of the gene are not limited to two alleles Where one gene may determine more than one trait Multifactorial inheritance in which the phenotype arises from the interaction of one or more genes with the environment, chance, and each other.
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3 Summary of dominance relationships Fig. 3.2
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4 Incomplete dominance in snapdragons Fig. 3.3
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5 Codominant lentil coat patterns Fig. 3.4a
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6 Codominant blood group alleles Fig. 3.4b
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7 Do variations on dominance relations negate Mendel’s law of segregation? Dominance relations affect phenotype and have no bearing on the segregation of alleles. Alleles still segregate randomly. Gene products control expression of phenotypes differently. Mendel’s law of segregation still applies. Interpretation of phenotype/genotype relation is more complex.
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8 How do we establish dominance relations between multiple alleles? Perform reciprocal crosses between pure breeding lines of all phenotypes Fig. 3.6
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9 Mutations are the source of new alleles Multiple alleles arise spontaneously in nature due to chance alterations in genetic material – mutations. Mutation rate varies from 1 in 10,000 to 1 in 1,000,000 per gamete per generation. Allele frequency is the percentage of the total number of gene copies represented by one allele. Wild-type – allele whose frequency is more than 1% Mutant allele – allele whose frequency is less than 1%
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10 Pleitropy – inheritance of coat color in mice a. Inbred agouti X yellow yields 1:1 agouti:yellow Yellow must be A Y A and A Y is dominant to A B. Yellow x yellow mice do not breed true. A Y is a recessive lethal. A Y A Y die in utero and do not Fig. 3.9
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11 Sickle-cell syndrome Multiple alleles Normal wild-type is Hb β A More than 400 mutant alleles identified so far Hb S allele specifies abnormal peptide causing sickling among red blood cells Pleitropy Hb S affects more than one trait Sickling Resistance to malaria Recessive lethality Different dominance relations
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12 Pleitropy of Sickle-cell syndrome Fig. 3.10
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13 Two genes can interact to determine one trait Novel phenotypes can emerge from the combination of alleles of two genes. Cross of tan and gray lentils produce
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extMendGenetics - Extensions to Mendel Complexities in...

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