What is the abnormal chromosome unique to genetic disorders such as Down syndrome or Turner syndrome?

Chromosomal Abnormalities in Genetic Disorders

Down Syndrome

Down syndrome is caused by trisomy 21, an extra copy of chromosome 21, making it the most common chromosomal abnormality associated with intellectual disability and congenital heart defects. 1

• The chromosomal abnormality consists of three copies of chromosome 21 instead of the normal two copies 1
• Approximately 40-50% of individuals with Down syndrome have congenital heart disease, most commonly atrioventricular septal defects (AVSD), ventricular septal defects (VSD), atrial septal defects (ASD), and tetralogy of Fallot 1
• Clinical features include characteristic facial features, developmental disability, hypotonia, and a single palmar crease 1
• Down syndrome is recognized at birth in most cases due to distinctive physical features 2

Turner Syndrome

Turner syndrome results from complete or partial loss of the second X chromosome, with the most common karyotype being 45,X (monosomy X), affecting approximately 1 in 2,500 liveborn girls. 3

• The chromosomal abnormality involves either complete absence of one X chromosome (45,X) or structural abnormalities of the X chromosome, often in mosaic patterns 1, 3
• Approximately 30-50% of patients have cardiovascular defects, including bicuspid aortic valve (15-30%), aortic coarctation (7-18%), and ascending aortic dilation (33%) 1, 3
• Clinical features include short stature, webbed neck, lymphedema, primary amenorrhea, and ovarian failure 1, 3
• Turner syndrome often is not recognized until adolescence, unlike Down syndrome 2
• Approximately 10% of cases contain Y chromosome material, which increases the risk for gonadoblastoma and requires bilateral prophylactic gonadectomy 4

Klinefelter Syndrome

Klinefelter syndrome is characterized by a 47,XXY karyotype, representing an extra X chromosome in phenotypic males. 1

• The chromosomal abnormality consists of one or more extra X chromosomes in males (most commonly 47,XXY) 1
• Approximately 50% have congenital heart disease, including patent ductus arteriosus, ASD, and mitral valve prolapse 1
• Clinical features include tall stature, hypoplastic testes, delayed puberty, and developmental disability 1
• Many men with Klinefelter syndrome are never diagnosed, making it frequently underrecognized 2

Key Clinical Distinctions

Each syndrome has a unique chromosomal abnormality: trisomy 21 for Down syndrome, monosomy X (or X chromosome structural abnormalities) for Turner syndrome, and XXY for Klinefelter syndrome. 1, 2

• All three syndromes are associated with increased risk of congenital heart defects and require cardiovascular screening 1
• Genetic evaluation is recommended in all patients with congenital heart defects accompanied by extracardiac malformations or neurodevelopmental abnormalities 1
• Double aneuploidy (such as Down-Turner syndrome with both trisomy 21 and X chromosome abnormalities) is extremely rare but has been documented 5, 6, 7