Q.36 The karyotype of individual is written as 47, XX, +21 indicating:
47, XX, +21 Karyotype Indicates Down Syndrome
The karyotype 47, XX, +21 represents a female with trisomy 21 (extra chromosome 21), the defining chromosomal abnormality of Down syndrome, making option (2) correct.
Introduction
The karyotype 47, XX, +21 specifically identifies Down syndrome caused by nondisjunction of chromosome 21 during meiosis, resulting in 47 total chromosomes instead of the normal 46. This trisomy 21 condition affects ~1:700 births and represents 95% of Down syndrome cases. This guide decodes karyotype notation and syndrome distinctions for genetics exam mastery.
Karyotype Notation Breakdown
47, XX, +21:
-
47 = Total chromosomes (normal = 46)
-
XX = Female sex chromosomes
-
+21 = Extra chromosome 21 (trisomy)
Option Analysis
Trisomy 21 vs other aneuploidies:
-
(1) Edward Syndrome: Incorrect. Trisomy 18 (47, XX/XY, +18). Severe developmental abnormalities; ~5-10% survival past 1 year. Lemon-shaped skull, rocker-bottom feet.
-
(2) Down Syndrome: Correct. Trisomy 21 (47, XX/XY, +21). Characteristic facies (upslanting palpebral fissures, epicanthic folds), intellectual disability (IQ 50-70), congenital heart defects (40-50%).
-
(3) Turner Syndrome: Incorrect. Monosomy X (45, X). Short stature, webbed neck, streak ovaries, coarctation of aorta. 45 chromosomes, not 47.
-
(4) Klinefelter Syndrome: Incorrect. XXY (47, XXY). Tall stature, gynecomastia, small testes, infertility. 47 chromosomes but sex chromosome trisomy, not autosome.
Aneuploidy Comparison Table
| Syndrome | Karyotype | Chromosome Count | Key Features |
|---|---|---|---|
| Down | 47,XX/XY,+21 | 47 | Intellectual disability, heart defects |
| Edward | 47,XX/XY,+18 | 47 | Rocker-bottom feet, clenched fists |
| Patau | 47,XX/XY,+13 | 47 | Cleft lip/palate, polydactyly |
| Turner | 45,X | 45 | Short stature, amenorrhea |
| Klinefelter | 47,XXY | 47 | Gynecomastia, infertility |
Clinical Implications
Down Syndrome (Trisomy 21):
-
Intellectual: Mild-moderate (IQ 40-70)
-
Cardiac: AVSD (40%), VSD, PDA
-
Hematologic: Transient myeloproliferative disorder (10%)
-
Increased risk: Alzheimer’s, leukemia, hypothyroidism
Maternal age effect: Risk rises exponentially >35 years (1:350 at 35; 1:100 at 40).
Exam Relevance
GATE Life Sciences: “+21 = Down” universal mnemonic. Key distinction: 47 autosome (Down/Edward/Patau) vs sex chromosome (Turner/Klinefelter). XX = female always.
Trap avoidance: Edward = 18 (not 21); Turner = 45 (not 47).
Prenatal Diagnosis
Triple/quad screen → Amniocentesis/CVS → Karyotyping confirms 47,XX,+21.
Visualization: Extra small chromosome 21 (acrocentric) in karyotype spread.
