Molecular biology and genetics: chromosomal mosaicism: 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 · Molecular biology and genetics: chromosomal mosaicism

Sample item

A girl with mild features of Down syndrome has 47,XX,+21 in 30 percent of cultured blood cells and 46,XX in the rest. How did this arise?

AA balanced translocation in a grandparentBRobertsonian translocation in a parentCMitotic nondisjunction after fertilisationDMeiosis I nondisjunction in the oocyte

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

The answer and the reasoning

C. Mitotic nondisjunction after fertilisation

Mosaicism arises from a postzygotic error, such as mitotic nondisjunction, which produces two or more cell lines in one individual. The phenotype is often milder because normal cells are present. Meiotic errors give a uniform trisomy in all cells.

Option by option

A. A balanced translocation in a grandparent
A balanced translocation would be inherited and present in all cells.
B. Robertsonian translocation in a parent
A parental Robertsonian translocation gives a fused chromosome in all cells.
C. Mitotic nondisjunction after fertilisation (correct)
Correct. A postzygotic mitotic error produces a mixture of normal and trisomic cells.
D. Meiosis I nondisjunction in the oocyte
A meiosis I error would give trisomy in every cell.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is C, Mitotic nondisjunction after fertilisation. Mosaicism arises from a postzygotic error, such as mitotic nondisjunction, which produces two or more cell lines in one individual.

Why is "A balanced translocation in a grandparent" not the answer?

A balanced translocation would be inherited and present in all cells.

Why is "Robertsonian translocation in a parent" not the answer?

A parental Robertsonian translocation gives a fused chromosome in all cells.

Why is "Meiosis I nondisjunction in the oocyte" not the answer?

A meiosis I error would give trisomy in every cell.

What concept does this question test?

It tests Molecular biology and genetics: chromosomal mosaicism in Biochemistry. The phenotype is often milder because normal cells are present. Meiotic errors give a uniform trisomy in all cells.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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 Molecular biology and genetics: chromosomal mosaicism?

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.