• Down syndrome (trisomy 21)
  • Most common viable chromosomal disorder and most common cause of genetic intellectual disability. 95% of cases due to meiotic nondisjunction (↑ with advanced maternal age; from 1:1500 in women < 20 to 1:25 in women > 45 years old). 4% of cases due to unbalanced Robertsonian translocation, most typically between chromosomes 14 and 21. Only 1% of cases are due to postfertilization mitotic error (mosaicism)
  • Incidence 1:700.
  • First-trimester ultrasound commonly shows ↑ nuchal translucency and hypoplastic nasal bone.
  • Findings: intellectual disability, low-set ears, redundant skin on nape of neck, micrognathia, hypotonia, flat facies, prominent epicanthal folds, single palmar crease, gap between 1st 2 toes, Brushfield spots
  • Associated with
    • Early-onset Alzheimer disease (chromosome 21 codes for amyloid precursor protein)
    • ↑ risk of ALL and AML
    • Hypothyroidism
    • DM1
    • Obesity
    • Duodenal atresia
    • Hirschprung disease
    • Atrioventricular septal defect
    • Atlanto-axial instability
  • Drinking age (21).

 

• Edwards syndrome (trisomy 18)
  • Findings: PRINCE Edward—Prominent occiput, Rocker-bottom feet, Intellectual disability, Nondisjunction, Clenched fists (with overlapping fingers), low-set Ears, micrognathia (small jaw), congenital heart disease. Death usually occurs by age 1.
  • Incidence 1:8000.
  • Election age (18).
  • 2nd most common autosomal trisomy resulting in live birth (most common is Down syndrome).

• Patau syndrome (trisomy 13)
  • Maternal age >=35
  • 47, XX ,+ 13
  • Defect in fusion of prechordal mesoderm (midface, eyes, forebrain)
  • Findings: severe intellectual disability, rockerbottom feet, microphthalmia, microcephaly, cleft liP/Palate, holoProsencephaly, Polydactyly, cutis aPlasia (focal skin defect on scalp, congenital heart disease, Polycystic kidney disease, omphalocele. Death usually occurs by age 1.
  • Incidence 1:15,000.
  • Puberty (13)