ttrr the expected genotypic ratio

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Unformatted text preview: ir babies has a ¼ chance of being homozygous recessive and having CAH. The chance that two independent (non ­identical) babies both have the disease is the product of the independent probabilities: ¼ x ¼ = 1/16. 3. Coat color in horses may be due to incomplete dominance. Chestnut horses are genotypically HcHc, cremello horses are genetically HwHw, but heterozygotes (HcHw) exhibit a third phenotype, palomino. Eddy, a palomino horse, had a long and enduring relationship with his barn ­mate Frieda, a cremello horse, which produced many foals. What is the chance that the foal Frieda is expecting will also be a cremello? (1 pt for parental genotypes and 1 pt for correct expected outcome). Eddy C H Hw w c w W w Frieda H HH HH Hw Hc Hw HwHw There is a 50% (one half) chance the foal is a cremello (HWHW) and 50% chance the foal is a palomino. 4. In humans, the occurrence of vitamin ­D ­resistant rickets is due to low blood phosphorous levels and is considered sex ­linked to the X chromosome. A female affected by vitamin ­D ­resistant rickets marries a ‘normal’ (unaffected) man and produces both affected and unaffected sons and daughters. What is the expected phenotypic ratio for the rickets trait for any children the couple might consider having? (1 pt for correct cross and assignment of dominance to trait; 1 pt. for expected outcome) Because the trait is sex ­linked male offspring will be hemizygous for the rickets’ gene while females will have two alleles. If the trait is recessive, then the female has to be homozygous recessive and the male dominant: male R X Y female Xr XRXr XrY r R r r X XX X Y This hypothesis should result in no affected daughters; but all affected sons. This does not match the data given. If the trait is dominant then the female may be homozygous dominant or heterozygous and the male recessive: Xr Y Xr Y R R r R R R r X XX X Y X XX XRY XR XRXr XRY Xr XrXr XrY If the female is homozygous ALL sons and daughters should be affected. This prediction does not match the data. If the female is heterozygous, then you would expect ½ of sons and ½ of daughters to be affected by rickets. This matches the data. So the expected outcome would be: 1 affected daughter: 1 unaffected daughter: 1 affected son: 1 unaffected son. 5. Labrador retriever dogs have two genes that affect coat color. One gene, the B locus, codes for the intensity of eumelanin production in the dog’s coat. It has two alleles that produce either a black pigment (B) or a brown pigment (b). Another gene, the E gene, is epistatic to the B gene. The dominant E allele causes eumelanin to be deposited along the length of the dog’s hair shafts. The recessive e allele blocks the deposition of eumelanin in the hair, so only the yellow producing phaeomelanin pigment is deposited, resulting in yellow dogs. If you cross parents with the following genotypes, bbee x bbEe, what phenotypic ratio do you expect in their offspring? (1 pt for correct cross at each locus and 1 pt for expected outcome) The B and E loci assort independently, so the expected outcome at each locus could be calculated independently and then multiplied: b b b bb bb b bb bb Expected phenotypic ratio is all brown (1 brown). E e e Ee ee e Ee ee Expected phenotypic ratio 1 eumelanin deposition (E_) : 1 no eumelanin deposition (ee) Multiplying 1 brown x 1 eumelanin deposition : 1 no eumelanin deposition gives: 1 brown color deposition : 1 brown no color deposition (yellow) for a ratio of 1 brown : 1 yellow....
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This note was uploaded on 09/16/2013 for the course BIOLOGY 231 taught by Professor Petethollenbeck during the Fall '10 term at Purdue University.

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