BioC 3021 Case Study 2 Patient X has been diagnosed with a mild disease. Like many diseases, this disease occurs in both sporadic and familial forms. The sporadic form occurs randomly in a single individual, while the familial form is inherited. The familial form can be identified by looking for mutations in any of five genes that are associated with the disease. If a patient has a mutation in one of these five genes, they probably have the familial form, which could be passed on to their children. Patient X would like to know if they are a carrier of this disease. As their physician, you order a test in which the patient's DNA is extracted and each of the five genes sequenced. The results show that four of the patient's genes are wild type, while the fifth shows a K to R substitution at amino acid 72. Bearing in mind that there are many polymorphisms in human DNA, what do you advise your patient? (A polymorphism is a site where the DNA sequence differs between individuals but does NOT cause any phenotypic difference.) Answer: It is impossible to say for sure, but it seems unlikely that this mutation causes disease. 1. The genome sequence in the database is derived from a small number of individuals. We don?t know for sure that they are ?wild type?. For example, they might carry some undiagnosed genetic disease. 2. There is tremendous genetic variability among humans. There are hundreds of thousands of polymorphisms (sites where individuals differ in DNA sequence but which have no effect). 3. Finally, the amino acid change is not severe. Lys and arg are both basic, and have the same general shape. Most proteins can tolerate amino acid substitutions if the new amino acid is in the same class as the original (hydrophobic, polar, acidic, basic, aromatic, etc.).
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