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Inheritance patterns flashcards

12 high-yield MRCP Part 1 cards from the Intellect Medicos Basic sciences & statistics course. Try to answer each question before you open it — then study them properly with spaced repetition, which brings each card back just before you would forget it.

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1. Hunter syndrome (MPS II) is inherited in an ______ pattern, unlike Hurler syndrome (MPS I).

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   X-linked recessive

   Why? Hurler (alpha-L-iduronidase deficiency) is autosomal recessive and causes corneal clouding; Hunter (iduronate sulfatase deficiency) spares the cornea and causes aggressive behaviour.

2. A man with an X-linked recessive disorder has children with a woman who is not a carrier. What are the risks to his sons and daughters?

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   Sons: none affected (they inherit his Y). Daughters: all are obligate carriers.

   Why? He passes his only X to every daughter and his Y to every son, so there is no male-to-male transmission.

3. Which trinucleotide repeat disorder does NOT classically show anticipation?

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   Friedreich's ataxia (GAA repeat, autosomal recessive).

   Why? Anticipation (earlier onset and greater severity in each generation) is typical of Huntington's (CAG), myotonic dystrophy (CTG) and fragile X (CGG).

4. A 25-year-old woman has myoclonic seizures, ataxia and proximal weakness. Her mother and brother have milder similar symptoms; no one on her father's side is affected. Which investigation finding is characteristic of the likely diagnosis?

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   Muscle biopsy showing ragged red fibres on modified Gomori trichrome stain (MERRF).

   Why? MERRF is a mitochondrial disorder, so it passes only through the mother: sperm mitochondria are destroyed after fertilisation.

5. Two healthy parents are both carriers of cystic fibrosis (autosomal recessive). What is the risk that their next child is affected, and the chance that their unaffected child is a carrier?

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   1 in 4 (25%) for an affected child; an unaffected child has a 2 in 3 chance of being a carrier.

   Why? Offspring ratio is 1 non-carrier : 2 carriers : 1 affected. Once 'affected' is excluded, 2 of the remaining 3 are carriers.

6. In an X-linked dominant disorder, what are the risks to the children of an affected father and an unaffected mother?

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   All his daughters are affected; none of his sons are.

   Why? He gives his X to every daughter and his Y to every son.

7. A woman carries haemophilia A; her partner is unaffected. What is the risk for each son and each daughter?

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   Each son: 50% chance of being affected. Each daughter: 50% chance of being a carrier.

   Why? Each child has a 50% chance of inheriting her mutant X. Sons have no second X to compensate; daughters receive a normal X from their father.

8. ______ is the proportion of people with a genotype who show the phenotype at all; expressivity is how severely the phenotype shows among those affected.

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   Penetrance

   Why? BRCA1 shows incomplete penetrance (not all carriers get cancer); NF1 shows variable expressivity (café-au-lait spots only vs severe neurofibromas).

9. Penetrance is the proportion of people with a genotype who show the phenotype at all; ______ is how severely the phenotype shows among those affected.

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   expressivity

   Why? BRCA1 shows incomplete penetrance (not all carriers get cancer); NF1 shows variable expressivity (café-au-lait spots only vs severe neurofibromas).

10. Which repeat expansion causes Huntington's disease, and on which chromosome is the gene?

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    A CAG repeat in the HTT (huntingtin) gene on chromosome 4.

    Why? Huntington's is autosomal dominant; longer repeats cause earlier onset.

11. A 35-year-old man has frontal balding, bilateral cataracts, difficulty letting go after a handshake and testicular atrophy. What is the underlying genetic defect?

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    CTG repeat expansion in the DMPK gene (myotonic dystrophy type 1, autosomal dominant).

    Why? The expanded repeat disrupts RNA splicing of many genes, giving a multisystem disorder.

12. A 22-year-old man has painless, subacute loss of central vision in one eye, followed weeks later by the other eye. His maternal uncle had the same problem. What is the inheritance pattern of the likely condition?

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    Mitochondrial (maternal) inheritance — Leber hereditary optic neuropathy.

    Why? Mitochondrial DNA comes only from the mother, so affected men never pass it on.