1.11 Aneuploidy Syndromes (Down, Turner, Klinefelter, Edwards, Patau)
Key Takeaways
- Trisomy 21 (Down syndrome) is usually nondisjunction 47,XX,+21 or 47,XY,+21; Robertsonian translocation and mosaicism change recurrence counseling and parental karyotype recommendations
- Maternal-age–related meiotic nondisjunction risk rises steeply after the mid-30s for autosomal trisomies; age framing must still include that most babies with Down syndrome are born to younger mothers because birth rates are higher
- Turner syndrome (45,X and mosaics) features short stature, left-sided cardiac anomalies (coarctation/bicuspid aortic valve), renal anomalies, and ovarian dysgenesis with infertility risk
- Klinefelter syndrome (47,XXY) presents with tall stature, hypogonadism, azoospermia/infertility, and variable learning or psychosocial concerns—often diagnosed in adolescence or infertility workups
- Trisomy 18 (Edwards) and trisomy 13 (Patau) are associated with multiple congenital anomalies and high perinatal mortality; counseling centers on natural history, goals of care, and diagnostic confirmation (karyotype/CMA context)
1.11 Aneuploidy Syndromes (Down, Turner, Klinefelter, Edwards, Patau)
Quick Answer: Autosomal trisomies (21, 18, 13) and sex-chromosome aneuploidies (45,X; 47,XXY) are core CGC conditions. Know the karyotype, whether the mechanism is nondisjunction vs translocation vs mosaicism, how maternal age modifies autosomal trisomy risk, and the phenotype + counseling differences between viable sex-chromosome aneuploidy and lethal/near-lethal autosomal trisomies.
Domain 1C items rarely ask only “what is Down syndrome?” They ask you to interpret a karyotype string, explain recurrence after a translocation finding, counsel after NIPT/ultrasound, or distinguish mosaic from non-mosaic implications.
Mechanisms You Must Separate
| Mechanism | Typical karyotype example | Counseling consequence |
|---|---|---|
| Meiotic nondisjunction | 47,XX,+21 | Recurrence slightly above population age-related risk; parental karyotypes usually normal |
| Robertsonian translocation | 46,XX,der(14;21)(q10;q10),+21 | Parental karyotyping required; recurrence can be substantially higher if a parent is a balanced carrier |
| Mosaicism | 47,XX,+21/46,XX | Phenotype often milder/variable; prenatal mosaic results need careful confirmation strategy |
| Sex-chromosome aneuploidy | 45,X or 47,XXY | Different natural history and fertility counseling than autosomal trisomy |
Mosaicism means two or more cell lines. Full aneuploidy implies essentially all examined cells share the aneuploid line (within laboratory limits). Do not equate “mosaic” with “benign”—severity phenotypes still occur—but expect greater variability and, prenatally, more interpretive complexity (including confined placental mosaicism after CVS).
Maternal-Age Risk Framing
Autosomal trisomy risk from meiotic nondisjunction rises with maternal age, especially after the mid-30s, because of longer oocyte arrest and increased meiotic error. Exam-ready counseling points:
- Quote age-specific risks from current validated tables used in your practice setting—not memorized folklore numbers that drift over decades.
- Still teach the epidemiology paradox: most infants with Down syndrome are born to women under 35 because more births occur in that age band.
- Paternal age is not the primary driver of classic trisomy 21 nondisjunction counseling the way maternal age is.
- Sex-chromosome aneuploidies have different age associations; do not automatically paste “advanced maternal age” scripts onto every 45,X or 47,XXY vignette.
Prenatal pathways (screening → diagnostic testing) belong with Domain 3 detail, but phenotype and karyotype literacy here is what makes those conversations coherent.
Down Syndrome (Trisomy 21)
Karyotype findings: ~95% are free trisomy 21 (47,XX,+21 or 47,XY,+21); ~2–3% Robertsonian translocation involving chromosome 21; ~2% mosaic.
| Domain | Essentials |
|---|---|
| Craniofacial / physical | Upslanting palpebral fissures, epicanthal folds, flat nasal bridge, single transverse palmar crease, sandal-gap toes, hypotonia |
| Cardiac | Congenital heart disease common; atrioventricular septal defect (AVSD) is classic |
| Development | Intellectual disability (variable); early intervention improves functional outcomes |
| Medical surveillance | Hypothyroidism, hearing/vision issues, atlantoaxial instability awareness, hematologic risk (including leukemia predisposition), early-onset Alzheimer disease in adulthood |
| Reproduction | Reduced fertility, especially in males; reproductive counseling individualized |
Translocation Down syndrome counseling trap: If the karyotype shows an unbalanced Robertsonian translocation, both parents should be offered karyotyping. A parent who is a balanced Robertsonian carrier has elevated recurrence risk; empiric risks depend on which parent carries the translocation and which chromosomes are involved. A de novo translocation in the child still warrants parental studies to confirm de novo status.
Prenatal counseling anchors: Increased NT, absent/hypoplastic nasal bone, abnormal serum analytes, or high-risk NIPT → offer diagnostic testing; discuss phenotype spectrum, medical comorbidities, and supports—not only “IQ number.” Postnatal counseling adds immediate newborn evaluation (echo, feeding/hypotonia, hematology) and connection to early intervention and syndrome-specific guidelines.
Turner Syndrome (Monosomy X Spectrum)
Karyotype: Classic 45,X; many individuals are mosaic (45,X/46,XX; 45,X/46,XY; structural X anomalies). Mosaic 45,X/46,XY raises distinct gonadal tumor surveillance issues—do not collapse all Turner counseling into one script.
| Feature cluster | Clinical notes |
|---|---|
| Growth | Short stature; growth hormone often considered |
| Lymphatic / neck | Cystic hygroma prenatally; webbed neck, low posterior hairline postnatally |
| Cardiac | Left-sided lesions: bicuspid aortic valve, coarctation of the aorta; lifelong aortopathy awareness |
| Renal | Horseshoe kidney and collecting-system anomalies |
| Gonadal | Ovarian dysgenesis → primary amenorrhea, infertility; hormone replacement planning |
| Cognition | Typically not intellectual disability of Down-syndrome type; may have specific learning (e.g., visuospatial) differences |
Prenatal: cystic hygroma, hydrops, cardiac/renal anomalies, or NIPT sex-chromosome flag. Postnatal/adolescent: short stature, delayed puberty, amenorrhea. Counsel cardiac imaging surveillance and reproductive options (egg donation, etc.) with sensitive infertility framing.
Klinefelter Syndrome (47,XXY)
| Feature | Counseling relevance |
|---|---|
| Habitus | Taller stature, longer limbs; may be subtle in childhood |
| Gonadal | Small testes, hypogonadism, azoospermia/infertility common |
| Endocrine | Testosterone deficiency → bone density, energy, sexual function concerns |
| Development | Language/learning differences, psychosocial vulnerability in some |
| Diagnosis timing | Infancy (hypospadias/developmental clues), school age, or adult infertility clinic |
Variants (48,XXXY, mosaics) exist; severity often correlates with additional X material. Prenatal 47,XXY counseling emphasizes the wide phenotypic range and available developmental/endocrine supports—avoid deterministic “always infertile / always disabled” language.
Edwards Syndrome (Trisomy 18) and Patau Syndrome (Trisomy 13)
| Trisomy 18 (Edwards) | Trisomy 13 (Patau) | |
|---|---|---|
| Karyotype | 47,XX,+18 or 47,XY,+18 (± mosaic/translocation rare mechanisms) | 47,XX,+13 or 47,XY,+13 (± Robertsonian involving 13) |
| Growth | Severe IUGR common | IUGR common |
| Hallmark anomalies | Clenched hands, overlapping fingers, rocker-bottom feet, omphalocele, CHD, choroid plexus cysts (soft marker context) | Holoprosencephaly spectrum, cleft lip/palate, postaxial polydactyly, ocular anomalies, CHD |
| Prognosis | High miscarriage/stillbirth and early mortality; rare longer-term survivors with profound disability | Similarly high perinatal mortality; profound disability in survivors |
| Counseling focus | Goals-of-care, accurate diagnostic confirmation, recurrence (usually age-related nondisjunction unless translocation) | Same; parental karyotypes if translocation mechanism |
Do not soft-pedal natural history, and do not strip autonomy: present medical facts, variability (especially mosaicism), and palliative versus interventional pathways as value-laden choices for the family.
Comparative Snapshot for Exam Recognition
| Syndrome | Karyotype cue | Highest-yield phenotype anchors |
|---|---|---|
| Down | +21 | Hypotonia, AVSD, characteristic facies, ID |
| Turner | 45,X (± mosaic) | Short stature, coarctation/BAV, ovarian dysgenesis |
| Klinefelter | 47,XXY | Tall, hypogonadism, infertility |
| Edwards | +18 | Clenched hands, rocker-bottom feet, severe anomalies, poor survival |
| Patau | +13 | Holoprosencephaly, cleft, polydactyly, poor survival |
Putting It Together in a Session
- Name the chromosome finding in plain language and technical karyotype terms.
- State whether mechanism is likely nondisjunction, translocation, or mosaic—and what parental testing that implies.
- Frame age-related risk honestly for autosomal trisomy without shaming reproductive timing.
- Match prenatal vs postnatal agendas (pregnancy decision-making and anomaly counseling vs newborn/childhood surveillance and school supports).
- Separate sex-chromosome aneuploidy natural history from trisomy 18/13 lethality narratives.
Mastering these five syndromes is the scaffold for later microarray and imprinting differentials: when the phenotype is “chromosomal” but the karyotype is normal, you move to CMA and methylation pathways—not to repeating a trisomy lecture.
A newborn with Down syndrome has karyotype 46,XX,der(14;21)(q10;q10),+21. What is the most important next counseling step regarding recurrence?
Which statement best reflects accurate maternal-age counseling for trisomy 21?
A 15-year-old with short stature, primary amenorrhea, and a bicuspid aortic valve is most likely to have which karyotype-related diagnosis among the classic aneuploidies?
Compared with non-mosaic trisomy 18, mosaic trisomy 18 detected on diagnostic testing most accurately implies which counseling point?