Identification: Bombay phenotype in paternity: practice question

A Forensic Medicine 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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Forensic Medicine · Identification: Bombay phenotype in paternity

Sample item

A man of blood group A is alleged to be the father of a group B child. The mother types as group O, but her serum also agglutinates group O red cells. Which explanation allows the man to be the father?

AThe mother has the Bombay phenotype and carries a hidden B geneBThe man has the cis-AB phenotypeCThe child has undergone a mutation of the ABO geneDThe man is a weak subgroup A2

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

The answer and the reasoning

A. The mother has the Bombay phenotype and carries a hidden B gene

A Bombay (hh) person lacks H substance, so red cells type as O even when A or B genes are present, and the serum contains anti-H that agglutinates O cells. Such a mother can transmit a B gene to the child. A group A man cannot give B, so only the mother's hidden gene explains it.

Option by option

A. The mother has the Bombay phenotype and carries a hidden B gene (correct)
Correct. A Bombay mother lacks H, types as O, agglutinates O cells and can pass a hidden B gene.
B. The man has the cis-AB phenotype
A cis-AB father would type as AB or A with weak B and does not fit a typed group A man.
C. The child has undergone a mutation of the ABO gene
A new mutation producing B is extremely rare and not an accepted explanation here.
D. The man is a weak subgroup A2
A weak A2 subgroup gives no B gene to pass on.

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.

Questions about this Forensic Medicine question

What is the answer to this Forensic Medicine question?

The best answer is A, The mother has the Bombay phenotype and carries a hidden B gene. A Bombay (hh) person lacks H substance, so red cells type as O even when A or B genes are present, and the serum contains anti-H that agglutinates O cells.

Why is "The man has the cis-AB phenotype" not the answer?

A cis-AB father would type as AB or A with weak B and does not fit a typed group A man.

Why is "The child has undergone a mutation of the ABO gene" not the answer?

A new mutation producing B is extremely rare and not an accepted explanation here.

Why is "The man is a weak subgroup A2" not the answer?

A weak A2 subgroup gives no B gene to pass on.

What concept does this question test?

It tests Identification: Bombay phenotype in paternity in Forensic Medicine. Such a mother can transmit a B gene to the child. A group A man cannot give B, so only the mother's hidden gene explains it.

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 Identification: Bombay phenotype in paternity?

Learn postmortem changes with their timelines, and toxicology as poison, sign, test and antidote. Then use the flashcards and the other Forensic Medicine 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.