Instructions

A hypothetical genetic disease is caused by a shortage of enzyme A. Enzyme A is involved in the formation of skin. Affected individuals have very stretchy, fragile skin that bruises easily. Mac McCool and Mindy Kool, who are phenotypically normal, are planning to start a family. They visit a genetic counselor and provide the following Information: Mac’s parents have normal skin, and so do his three older sisters. Mac’s father has an older sister who has the disease. Mac’s grandparents on his father’s side have normal skin. Mindy’s parents have normal skin, but she has a younger brother who has the syndrome. Mindy’s father has two younger sisters who are non-identical twins and have normal skin. Mindy’s grandparents on her father’s side have normal skin. 1.1 a. Draw a pedigree using conventional symbols and numbering to show three generations of the McCools and the Kools. (3) b. Using the symbols A/a, assign genotypes as far as possible to Mac’s father and Mac’s father’s sister. (2) c What is the probability that Mac and Mindy’s first child wil have the syndrome? Assume Mac’s mother is homozygous for normal skin. (Previse show all your calculations). (2) Molecular studies performed on Mindy’s affected brother have identified a mutation in the gene coding for enzyme A. The following shows the sequence of part of a wild-type ailele and a mutant allele A of the gene, including the start codon. The bottom strand is the template for transcription. Wild-type allele for enzyme A Mutant allele for enzyme A 5 TTATGCOCAACATCCCA 3 50 TIATGGGCTAGATCOCA 3 31 AATACOCCITCTACGCT S’ 30 AATACOCCATCTAGOOT 5 1.2 a. What mutation has occurred? 6. How does this mutation affect the enzyme? (1) (1) 1.3 In other studies of this syndrome, it has been found that individuals that have both the original mutant allele for enzyme A shown above AND a mutant allele for a gene that encodes a lysine RNA are able to make some normal enzyme A The mutant lysine RNA allele makes a tRNA that has one base that is different from the wild-type RNA. Explain how the lysine tRNA mutation allows synthesis of the normal enzyme A. (2) 1.4 If it is found that the absence of enzyme A can lead to cancer, what type of cancer-causing gene is the gene coding for enzyme A? (1)

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