Robertsonian Translocation Down Syndrome — Carrier Parent
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baby OBG
A 28-year-old primigravida delivers a baby boy with characteristic features of Down syndrome including flat facial profile, upslanting palpebral fissures, brachycephaly, single transverse palmar crease, and hypotonia. Echocardiography confirms a complete atrioventricular septal defect. The neonate's karyotype is 46,XY,der(14;21)(q10;q10),+21. The structure marked **B** in the diagram represents the mother's karyotype: 45,XX,der(14;21)(q10;q10). Which of the following is the MOST IMPORTANT next step in genetic counseling for this family?
A. Advise the mother that advanced maternal age is the primary risk factor and recommend delaying future pregnancies until after age 35
B. Recommend immediate karyotyping of the mother's siblings and counsel them that they have a 50% chance of being carriers of the same translocation
Inform the mother that her recurrence risk for future pregnancies is approximately 10–15% and offer prenatal diagnosis (CVS or amniocentesis) or preimplantation genetic testing with IVF
C.
D. Reassure the mother that her balanced translocation is a de novo event and the recurrence risk is <1% in future pregnancies
Explanation
Why option 1 is correct
The mother carries a balanced Robertsonian translocation der(14;21), with only 45 chromosomes but a phenotypically normal phenotype because the lost short-arm material contains only redundant rRNA genes. The critical clinical anchor is that maternal carriers of der(14;21) have an empirical recurrence risk of approximately 10–15% for liveborn Down syndrome in future pregnancies—substantially higher than the population risk. This risk arises because segregation of the unbalanced translocation during meiosis can produce gametes carrying both the fused chromosome and a normal 21, resulting in trisomy 21 in the offspring. Genetic counseling must include discussion of prenatal diagnosis options (CVS at 11 weeks or amniocentesis at 16 weeks) and preimplantation genetic testing (PGT) with IVF as an alternative reproductive strategy. This is the standard of care per Williams Obstetrics 26e.
Why each distractor is wrong
Option 2: While some translocation Down syndrome cases are de novo, this case is NOT de novo—the mother carries the balanced translocation. Assuming a de novo event would be incorrect and would dangerously underestimate recurrence risk. The presence of the balanced carrier parent is the defining feature that mandates parental karyotyping and changes counseling entirely.
Option 3: Advanced maternal age is NOT a relevant risk factor in translocation Down syndrome, unlike free trisomy 21 (where maternal age is the major risk). The recurrence risk in translocation carriers is independent of maternal age and is determined by the segregation pattern of the unbalanced translocation during meiosis.
Option 4: While cascade karyotyping of the mother's siblings IS part of comprehensive counseling (to identify other carriers and offer them counseling), this is a secondary step. The immediate priority is to counsel the proband's parents about their own recurrence risk and reproductive options, not to screen the extended family first.
High-YieldNEET PG
Maternal carriers of der(14;21) Robertsonian translocation have 10–15% recurrence risk for translocation Down syndrome; paternal carriers have 1–2% risk; der(21;21) carriers have ~100% risk. Maternal age is irrelevant—segregation pattern is everything.
Williams Obstetrics 26e — Genetic counseling; Robertsonian translocation Down syndrome
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