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Testing Your Knowledge
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1. How might pedigrees for an autosomal recessive and an autosomal dominant trait differ?

2. Mel and Barbara are healthy, but they know from a blood test that they are each carriers of Tay-Sachs disease. If their first three children are healthy, what is the probability that their fourth child will inherit the disease?

3. Calculate the genotypic and phenotypic ratios for the following crosses in peas for height, determined by the T gene:

  • a. Homozygous dominant crossed with homozygous recessive.
  • b. Homozygous dominant crossed with heterozygous.
  • c. Homozygous recessive crossed with heterozygous.
  • 4. This unusual pedigree depicts the Ptolemy dynasty, which began in 323 B.C. and ended with Cleopatra in 30 B.C. What is unusual about the pedigree? (refer to art in the text)

    5. Which genetic principle does each of the following examples illustrate?

  • a. Many different genes affect hearing in humans. Being homozygous recessive for certain types of these genes results in deafness, even if the other hearing genes are wild type.
  • b. A woman develops dark patches on her face and her physician suggests that she has alkaptonuria, an inherited illness. However, a dermatologist discovers that the woman’s skin problem is due to using a facial cream that contains a chemical known to cause dark patches in people with dark skin.
  • 6. In cats with the Manx trait, the M allele causes a short or absent tail whereas the m allele confers a normal, long tail. Cats of genotype MM die as embryos. If two Manx cats mate, what is the probability that each living kitten has a long tail?

    7. The autosomal dominant disorder, distal symphalangism, causes the fingers and toes to be short with tiny nails. How can this disorder be both incompletely penetrant and variably expressive?

    8. Domesticated hens with white feathers and large, single combs mate with roosters that have dark feathers and small combs. The offspring all resemble their mothers for these two traits.

  • a. Which alleles of each trait are dominant, and which are recessive?
  • b. If the F1 were crossed with each other, what percentage of the F2 would be expected to have dark feathers and large, single combs?
  • 9. Two genes carried on different chromosomes impart pigment to potato skin. The D gene provides red color, and the P gene provides blue. F1, the dominant allele of the D gene, is present, however, the potatoes are red, no matter which P gene allele is present. Which phenomenon that can alter Mendelian ratios does this illustrate?

    10. Define complex, multifactorial, and polygenic traits.

    11. In cats, several genes control coat color. If a dominant allele is present for either the white gene (w) or the orange gene (O), the other pigment genes are not expressed. What phenomenon does this illustrate?

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