A 31-year-old woman presents with three first-trimester miscarriages (at 8, 10, and 9 weeks gestation). She is phenotypically normal with no medical comorbidities. Antiphospholipid antibodies and uterine cavity imaging are normal. Karyotyping reveals the abnormality marked **A** in the diagram: 46,XX,t(11;22)(q23;q11.2). Her husband's karyotype is normal (46,XY). Which of the following best explains the mechanism of recurrent pregnancy loss in this carrier?
A. During meiosis, adjacent-1, adjacent-2, and 3:1 segregation of the quadrivalent produce unbalanced gametes that result in non-viable or severely affected conceptuses
B. Homozygous carriers of the same translocation have a 100% risk of miscarriage due to double translocation burden
C. The translocation disrupts the centromere structure, preventing normal chromosome segregation in all meiotic divisions
D. The balanced translocation itself causes a gene dosage imbalance that is incompatible with early embryonic development
Explanation
Why "During meiosis, adjacent-1, adjacent-2, and 3:1 segregation of the quadrivalent produce unbalanced gametes that result in non-viable or severely affected conceptuses" is right
The balanced reciprocal translocation marked A (46,XX,t(11;22)(q23;q11.2)) is the most common recurrent constitutional reciprocal translocation in humans. During meiosis, the translocated chromosomes form a quadrivalent that can segregate in multiple ways. Alternate segregation produces balanced gametes (either normal or carrier offspring), but adjacent-1, adjacent-2, and 3:1 segregations produce unbalanced gametes with partial monosomy and partial trisomy. These unbalanced products are non-viable or result in severe conditions like Emanuel syndrome (from 3:1 segregation), explaining the recurrent first-trimester losses. The carrier herself is phenotypically normal because she has balanced genetic material (Smith's Recognizable Patterns — Translocations).
Why each distractor is wrong
The balanced translocation itself causes a gene dosage imbalance that is incompatible with early embryonic development: This contradicts the definition of a balanced translocation. By definition, the carrier has no net gain or loss of genetic material and is phenotypically normal. Gene dosage imbalance arises only in unbalanced offspring, not in the carrier.
Homozygous carriers of the same translocation have a 100% risk of miscarriage due to double translocation burden: This is incorrect. Homozygous carriers are extremely rare and are not the issue here. The woman is a heterozygous carrier (46,XX,t(11;22)), and her risk of miscarriage is ~50–70% per conception due to unbalanced segregation products, not 100%.
The translocation disrupts the centromere structure, preventing normal chromosome segregation in all meiotic divisions: This is false. The translocation does not disrupt centromere function; segregation occurs normally in some meiotic divisions (alternate segregation). The problem is that abnormal segregation patterns (adjacent-1, adjacent-2, 3:1) produce unbalanced gametes, not that all segregation is abnormal.
High-YieldNEET PG
Balanced reciprocal translocation carriers are phenotypically normal but face ~50–70% miscarriage risk due to unbalanced gamete production; preimplantation genetic testing (PGT-SR) or prenatal diagnosis (CVS/amniocentesis) are counseling options.
Smith's Recognizable Patterns — Translocations
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