Nondisjunction aneuploidy vs Robertsonian translocation and mosaicism
How to tell Nondisjunction aneuploidy and Robertsonian translocation and mosaicism apart when a board question makes them look alike.
Nondisjunction changes chromosome number; Robertsonian rearranges acrocentric long arms; mosaicism means two postzygotic cell lines.
Side by side
Nondisjunction causes whole-chromosome gain or loss, usually sporadic, with risk increasing with maternal age and findings like trisomy or monosomy.
Robertsonian translocation fuses long arms of acrocentric chromosomes and can cause familial Down syndrome; mosaicism creates two cell lines after postzygotic mitotic error.
Meiotic chromosome separation failure produces gametes with abnormal chromosome number.
Balanced translocation carriers have rearranged chromosomes with reproductive risk, while mosaicism arises after fertilization and gives variable tissue involvement.
Full trisomy with older maternal age and no carrier parent supports nondisjunction.
Normal chromosome count with translocation material supports Robertsonian; mixed normal/abnormal cell lines support mosaicism.
The trap
Down syndrome can come from nondisjunction, Robertsonian translocation, or mosaicism, so karyotype mechanism matters.
The repair move
For chromosomal questions, read the karyotype mechanism before relying on the syndrome name.
Try a free question
CoreA 42-year-old pregnant woman has chorionic villus sampling showing a fetal karyotype of 47,XX,+21. Both parents have normal karyotypes (46,XX and 46,XY). Which mechanism best explains the fetus's trisomy 21?
Show answer and explanation
Answer: A. Maternal meiotic nondisjunction. Most trisomy 21 (about 95%) results from maternal meiotic nondisjunction; risk rises sharply with advancing maternal age. The full extra chromosome 21 produces a 47-chromosome karyotype.
Why the other choices are tempting
- B. A Robertsonian translocation in a parent would show abnormal parental karyotype, but both parents are normal here.
- C. Mosaicism would show two cell lines (mixed normal and trisomic), not a uniform 47,+21 karyotype.
- D. Uniparental disomy involves two copies of a chromosome from one parent, not three total copies.
- E. Imprinting defects (Prader-Willi/Angelman) affect chromosome 15 expression, not chromosome 21 trisomy.
Flashcards
Tap a card to flip it.
What clue supports Nondisjunction aneuploidy?
Choose Nondisjunction aneuploidy when the stem gives this pattern: Nondisjunction causes whole-chromosome gain or loss, usually sporadic, with risk increasing with maternal age and findings like trisomy or monosomy.
What clue supports Robertsonian translocation and mosaicism?
Choose Robertsonian translocation and mosaicism when the stem gives this contrasting pattern: Robertsonian translocation fuses long arms of acrocentric chromosomes and can cause familial Down syndrome; mosaicism creates two cell lines after postzygotic mitotic error.
Quick answers
What is the fastest way to tell Nondisjunction aneuploidy from Robertsonian translocation and mosaicism?
Nondisjunction changes chromosome number; Robertsonian rearranges acrocentric long arms; mosaicism means two postzygotic cell lines.
What is the key clue for Nondisjunction aneuploidy?
Nondisjunction causes whole-chromosome gain or loss, usually sporadic, with risk increasing with maternal age and findings like trisomy or monosomy.
What is the key clue for Robertsonian translocation and mosaicism?
Robertsonian translocation fuses long arms of acrocentric chromosomes and can cause familial Down syndrome; mosaicism creates two cell lines after postzygotic mitotic error.
What is the common trap with Nondisjunction aneuploidy vs Robertsonian translocation and mosaicism?
Down syndrome can come from nondisjunction, Robertsonian translocation, or mosaicism, so karyotype mechanism matters.