24 imagine that a newly discovered recessively

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24) Imagine that a newly discovered, recessively inherited disease is expressed only in individuals with type O blood, although the disease and blood group are independently inherited. A normal man with type A blood and a normal woman with type B blood have already had one child with the disease. The woman is now pregnant for a second time. What is the probability
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24 that the second child will also have the disease? Assume that both parents are heterozygous for the gene that causes the disease. Answer: 1/16 25) In tigers, a recessive allele causes an absence of fur pigmentation (a white tiger) and a cross- eyed condition. If two phenotypically normal tigers that are heterozygous at this locus are mated, what percentage of their offspring will be cross-eyed? What percentage will be white? Answer: 25% will be cross-eyed; all of the cross-eyed offspring will also be white. 26) In corn plants, a dominant allele I inhibits kernel color, while the recessive allele i permits color when homozygous. At a different locus, the dominant allele P causes purple kernel color, while the homozygous recessive genotype pp causes red kernels. If plants heterozygous at both loci are crossed, what will be the phenotypic ratio of the offspring? Answer: The dominant allele I is epistatic to the P/p locus, and thus the genotypic ratio for the F 1 generation will be 9 I_P _ (colorless) : 3 I_pp (colorless) : 3 iiP _ (purple) : 1 iipp (red). Overall, the phenotypic ratio is 12 colorless : 3 purple : 1 red. 27) The pedigree below traces the inheritance of alkaptonuria, a biochemical disorder. Affected individuals, indicated here by the colored circles and squares, are unable to metabolize a substance called alkapton, which colors the urine and stains body tissues. Does alkaptonuria appear to be caused by a dominant allele or by a recessive allele? Fill in the genotypes of the individuals whose genotypes can be deduced. What genotypes are possible for each of the other individuals? Answer: Recessive. All affected individuals (Arlene, Tom, Wilma, and Carla) are homozygous recessive aa . George is Aa , since some of his children with Arlene are affected. Sam, Ann, Daniel, and Alan are each Aa , since they are all unaffected children with one affected parent. Michael also is Aa , since he has an affected child (Carla) with his heterozygous wife Ann. Sandra, Tina, and Christopher can each have the AA or Aa genotype. 28) A man has six fingers on each hand and six toes on each foot. His wife and their daughter have the normal number of digits. Extra digits is a dominant trait. What fraction of this couple's children would be expected to have extra digits? Answer: 1/2
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25 29) Imagine that you are a genetic counselor, and a couple planning to start a family comes to you for information. Charles was married once before, and he and his first wife had a child with cystic fibrosis. The brother of his current wife, Elaine, died of cystic fibrosis. What is the probability that Charles and Elaine will have a baby with cystic fibrosis? (Neither Charles nor Elaine has cystic fibrosis.) Answer: 1/6
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