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lecture_13__05 - Genetics is not as simple as Mendel...

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Genetics is not as simple as Mendel proposed... a few common deviations from simple dominant vs. recessive alleles: incomplete dominance : heterozygotes with two different alleles have a phenotype intermediate between the homozygous phenotypes Fig. 14.9 homozygotes recovered in the F2 generation exhibit the original parental phenotypes
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codominance : heterozygotes with two different alleles exhibit the phenotypic traits of both the parental homozygotes NN MN the phenotype of the AB heterozygote in this case is a combination of the homozygous phenotypes and is not intermediate between them. (This can often depend on the way the phenotype is measured). Example: blood type 3 different alleles at the I locus: I A , I B , and i
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Couple# 1 2 3 blood groups: A and A A and B B and O blood types of babies: B, O, AB Which couples match with which babies? Couple# 4 5 6 blood groups: A and AB A and O AB and O blood types of babies: A, B, AB genotypes?
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In many cases, trait may appear to be recessive in terms of phenotype, but in fact shows incomplete dominance at the biochemical level Example: Disease X is caused by mutations in a gene encoding an enzyme. The symptoms of the disease are only apparent in homozygous recessive individuals and not in heterozygotes. Yet when the levels of enzyme activity are measured, the heterozygotes only have half the level of the homozygous dominant individuals. genotype: AA Aa aa enzyme activity: 100 50 0 disease symptoms: no no yes Why does the heterozygote not exhibit symptoms? Because there is a threshold of enzyme activity which is necessary for normal function. In this case, the level of activity in the heterozygote is above this threshold and is therefore phenotypically normal. This is a common scenario for homozygous recessive disorders.
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In cattle, roan coat color (mixed red and white hairs) occurs in the heterozygous (Rr) offspring of red (RR) and white (rr) homozygotes. When two roan cattle are crossed,
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