A phenotypically normal woman whose sister had a child with Down syndrome undergoes karyotyping. She is found to carry a balanced robertsonian translocation involving chromosomes 14 and 21, 45,XX,t(14;21)(q10;q10). During gametogenesis, what gamete produced by adjacent segregation would result in a viable offspring with Down syndrome?

• A No viable gamete can transmit the translocation because carriers are always infertile
• B A gamete containing only the der(14;21) chromosome, giving 45 chromosomes after fertilization
• C A gamete containing only the free chromosome 21, giving 47 chromosomes after fertilization
• D A gamete containing both the der(14;21) chromosome and a free chromosome 21, giving 46 chromosomes after fertilization ✓

Explanation

D balanced rob(14;21) carrier has one der(14;21) plus one normal 14 and one normal 21. D gamete carrying the derivative chromosome together with a free chromosome 21 yields a zygote with two copies of the long arm of 21 on the derivative plus one additional free copy, producing translocation trisomy 21 with a normal chromosome number of 46. Such carriers have a significant recurrence risk, unlike parents of children with free trisomy 21, whose recurrence risk is near the age-related baseline.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.