Inheritance patterns: X-linked dominant: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Inheritance patterns: X-linked dominant

Sample item

In a family with hypophosphataemic rickets resistant to vitamin D, an affected man has five children: all three daughters are affected and both sons are unaffected. An affected woman in the next generation has both affected and unaffected children of both sexes. There is no male-to-male transmission. Which inheritance pattern fits best?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

D. X-linked dominant

An affected father passes his single X chromosome to all daughters and his Y to all sons, so every daughter is affected and no son is. An affected heterozygous mother transmits the allele to half of her children irrespective of sex. Absence of male-to-male transmission separates X-linked dominant from autosomal dominant inheritance, and the PHEX gene is mutated in this form of rickets.

Option by option

  • A. X-linked recessive

    X-linked recessive disease mainly affects males, and carrier daughters would be unaffected.

  • B. Mitochondrial

    Mitochondrial inheritance cannot pass from an affected father to his children.

  • C. Autosomal dominant

    Autosomal dominant inheritance would allow father-to-son transmission.

  • D. X-linked dominant (correct)

    All daughters and no sons of an affected father point to X-linked dominant inheritance.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is D, X-linked dominant. An affected father passes his single X chromosome to all daughters and his Y to all sons, so every daughter is affected and no son is.

Why is "X-linked recessive" not the answer?

X-linked recessive disease mainly affects males, and carrier daughters would be unaffected.

Why is "Mitochondrial" not the answer?

Mitochondrial inheritance cannot pass from an affected father to his children.

Why is "Autosomal dominant" not the answer?

Autosomal dominant inheritance would allow father-to-son transmission.

What concept does this question test?

It tests Inheritance patterns: X-linked dominant in Biochemistry. An affected heterozygous mother transmits the allele to half of her children irrespective of sex. Absence of male-to-male transmission separates X-linked dominant from autosomal dominant inheritance, and the PHEX gene is mutated in this form of rickets.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Inheritance patterns: X-linked dominant?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.