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{{WBRQuestion
{{WBRQuestion
|QuestionAuthor={{Rim}}
|QuestionAuthor= {{YD}} {{Alison}} (Reviewed by Serge Korjian)
|ExamType=USMLE Step 1
|ExamType=USMLE Step 1
|MainCategory=Genetics
|MainCategory=Genetics
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|MainCategory=Genetics
|MainCategory=Genetics
|SubCategory=Reproductive
|SubCategory=Reproductive
|MainCategory=Genetics
|MainCategory=Genetics
|MainCategory=Genetics
|MainCategory=Genetics
|MainCategory=Genetics
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|MainCategory=Genetics
|MainCategory=Genetics
|SubCategory=Reproductive
|SubCategory=Reproductive
|Prompt=A 2 month old male patient is brought by his parents to the pediatrician’s office for multiple fractures with minimal trauma.  The pediatrician explains to the family that the patient has osteogenesis imperfecta that is most commonly inherited in an autosomal dominant pattern.  Genetic analysis then reveals that the patient’s mother has mutant genotype of osteogenesis imperfecta, but she is not affected by the disease. Which genetic characteristic best explains this finding?
|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?
|Explanation=[[Osteogenesis imperfecta]] (OI) is a specific example of [[incomplete penetrance]].  The majority of OI cases has a [[dominant mutation]] in one of the two genes that produce type I collagen: COL1A1 or COL1A2.  [[Incomplete penetrance]] is defined as the unpredictable presence of mutant phenotype in individuals despite the confirmed presence of the mutant [[genotype]]. As such, not all patients who have the mutation will express it.  In this example, the child has the mutant [[genotype]] and expressed OI, whereas his mother also has the disease but did not express it and is thus not affected with OI.
|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 the unpredictable presence of mutant phenotype in individuals despite the confirmed presence of the mutant [[genotype]].  As such, not all patients who have the mutation will express it.
 
 
|AnswerA=Variable expression
|AnswerA=Variable expression
|AnswerAExp=Variable expression is the range of disease symptoms and severity from one person to another who in fact have the same mutation.  Neurofibromatosis type I is a classical example of variable expression.
|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.
|AnswerB=Incomplete penetrance
|AnswerB=Incomplete penetrance
|AnswerBExp=Incomplete penetrance is when not all people with the mutant genotype express the phenotype, such as OI.
|AnswerBExp=[[Incomplete penetrance]] is when not all individuals with a mutant genotype express the phenotype, such as OI.
|AnswerC=Locus heterogeneity
|AnswerC=Locus heterogeneity
|AnswerCExp=Locus heterogeneity is defined as several mutations in several genes may eventually have the same phenotype.  A classical example if Marfanoid habitus that is expressed with mutations of FBN1 gene that causes Marfan syndrome, and with genetic mutations that cause MEN2B syndrome and homocystinuria.
|AnswerCExp=Locus heterogeneity is defined as mutations at different locations that result in the same phenotype.
|AnswerD=Heteroplasmy
|AnswerD=Heteroplasmy
|AnswerDExp=Heteroplasmy is the presence of more than one mitochondrial DNA type that results in variable expression of mitochondrial inherited disease.
|AnswerDExp=Heteroplasmy is the presence of more than one mitochondrial DNA type, resulting in variable expression of a mitochondrial inherited disease.
|AnswerE=Imprinting
|AnswerE=Imprinting
|AnswerEExp=Imprinting is the situation when the phenotype of the patient depends on whether it is derived from the father or the mother. 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.
|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.
|EducationalObjectives=[[Incomplete penetrance]] is defined as is the presence of an ordinary phenotype in an individual despite the confirmed presence of a mutant [[genotype]].
|References=First Aid 2014 page 84
|RightAnswer=B
|RightAnswer=B
|WBRKeyword=incomplete, penetrance, osteogenesis, imperfecta, autosomal, dominant, mutation
|WBRKeyword=Incomplete penetrance, Osteogenesis imperfecta, Autosomal dominant, genetics, Inheritance,
|Approved=No
|Approved=Yes
}}
}}

Latest revision as of 00:33, 28 October 2020
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.
References: First Aid 2014 page 84]]

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::
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