WBR0436
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| Author | [[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Alison Leibowitz) (Reviewed by Serge Korjian)]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Genetics |
| Sub Category | SubCategory::Reproductive |
| Prompt | [[Prompt::A 2-month-old boy is brought the pediatrician’s office for a history of multiple fractures with minor trauma. Work-up reveals the patient has osteogenesis imperfecta. The pediatrician explains to the family that the disease is most commonly inherited in an autosomal dominant pattern. Genetic analysis of both parents reveals that the child’s mother has a genotype similar to that of her son, but is not affected by the disease. Which of the following genetic characteristics best explains this finding?]] |
| Answer A | AnswerA::Variable expression |
| Answer A Explanation | [[AnswerAExp::Variable expression is the range of disease symptoms and severity in individuals who have the same mutation. Neurofibromatosis type I is an example of variable expression.]] |
| Answer B | AnswerB::Incomplete penetrance |
| Answer B Explanation | [[AnswerBExp::Incomplete penetrance is when not all individuals with a mutant genotype express the phenotype, such as OI.]] |
| Answer C | AnswerC::Locus heterogeneity |
| Answer C Explanation | AnswerCExp::Locus heterogeneity is defined as mutations at different locations that result in the same phenotype. |
| Answer D | AnswerD::Heteroplasmy |
| Answer D Explanation | AnswerDExp::Heteroplasmy is the presence of more than one mitochondrial DNA type, resulting in variable expression of a mitochondrial inherited disease. |
| Answer E | AnswerE::Imprinting |
| Answer E Explanation | [[AnswerEExp::Imprinting is the situation when a phenotype is dependent on whether a particular gene is or is not methylated. For example, Prader-Willi and Angelman’s syndromes are both mutations of the same region in chromosome 15; but the disease is caused by a mutation derived paternally in Prader-Willi, whereas it is derived maternally in Angelman’s syndrome.]] |
| Right Answer | RightAnswer::B |
| Explanation | [[Explanation::Osteogenesis imperfecta (OI) is an example of incomplete penetrance. The majority of OI cases have a dominant mutation in one of the two genes that produce type I collagen: COL1A1 or COL1A2. Incomplete penetrance is the presence of an ordinary phenotype in an individual despite the confirmed presence of a mutant genotype. In this scenario, the child has a mutant genotype and expressed OI, whereas his mother also has the mutation in her genotype but did not express it phenotypically. Educational Objective: Incomplete penetrance is defined as is the presence of an ordinary phenotype in an individual despite the confirmed presence of a mutant genotype. |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::Incomplete penetrance, WBRKeyword::Osteogenesis imperfecta, WBRKeyword::Autosomal dominant, WBRKeyword::genetics, WBRKeyword::Inheritance |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |