Down Syndrome vs. Turner Syndrome: Chromosomal Differences & Clinical Overlap
Both are chromosomal aneuploidies with multi‑system effects, but they differ fundamentally in mechanism and phenotype.
Genetic Basis
| Feature | Down Syndrome (Trisomy 21) | Turner Syndrome (45,X / Mosaic) |
|---|
| Chromosome change | Extra chr 21 (95% free trisomy) | Monosomy X (50%) or mosaic (45,X/46,XX etc.) |
| Incidence | ~1:700 live births | ~1:2,000–2,500 female births |
| Sex ratio | Both sexes | Females only |
Shared Features
- Short stature
- Congenital heart defects (AVSD in DS; bicuspid aortic valve, coarctation in TS)
- Renal anomalies
- Thyroid dysfunction (hypothyroidism common in both)
- Increased risk of autoimmune conditions
- Prenatal diagnosis: NIPT, CVS, amnio
Distinctive Features
| Down Syndrome | Turner Syndrome |
|---|
| Intellectual disability (mild‑moderate) | Normal intelligence; specific NF‑1 profile (visuospatial, math) |
| Characteristic facies (upslanting palpebral fissures, epicanthal folds, flat profile) | Webbed neck, low posterior hairline, broad chest, widely spaced nipples |
| Single palmar crease, clinodactyly | Lymphedema hands/feet (neonates) |
| ↑ Leukemia (ALL, AML), Alzheimer's (early) | ↑ Osteoporosis, infertility (streak gonads), aortic dissection risk |
| Hypotonia, joint laxity | Normal tone; scoliosis, kyphosis |
Management
- DS: Multidisciplinary (cardiology, ENT, ophthalmology, GI, developmental, Alzheimer's surveillance).
- TS: Growth hormone (childhood), estrogen replacement (puberty), cardiac/renal/aortic monitoring, fertility counseling (oocyte donation).
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