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BIOCHEmISTRY ``BIOCHEMISTRY—GENETICS BIOCHEmISTRY ``BIOCHEMISTRY—GENETICS SECTION II 63

Autosomal trisomies
Down syndrome Findings: intellectual disability, flat facies, Incidence 1:700.
(trisomy 21) prominent epicanthal folds, single palmar Drinking age (21).
crease, incurved 5th finger, gap between 1st 2 Most common viable chromosomal disorder
toes, duodenal atresia, Hirschsprung disease, and most common cause of genetic
congenital heart disease (eg, ASD), Brushfield intellectual disability.
spots. Associated with early-onset Alzheimer First-trimester ultrasound commonly shows
disease (chromosome 21 codes for amyloid  nuchal translucency and hypoplastic nasal
precursor protein),  risk of AML/ALL. bone. Markers for Down syndrome are HI up:
95% of cases due to meiotic nondisjunction  hCG,  inhibin.
( with advanced maternal age; from 1:1500 in The 5 A’s of Down syndrome:
women < 20 to 1:25 in women > 45 years old). ƒ Advanced maternal age
Single palmar crease 4% of cases due to unbalanced Robertsonian ƒ Atresia (duodenal)
translocation, most typically between ƒ Atrioventricular septal defect
chromosomes 14 and 21. Only 1% of cases are ƒ Alzheimer disease (early onset)
due to postfertilization mitotic error. ƒ AML/ALL
Edwards syndrome Findings: PRINCE Edward—Prominent Incidence 1:8000.
(trisomy 18) occiput, Rocker-bottom feet, Intellectual Election age (18).
disability, Nondisjunction, Clenched fists with 2nd most common autosomal trisomy resulting
overlapping fingers, low-set Ears, micrognathia in live birth (most common is Down syndrome).
(small jaw), congenital heart disease,
omphalocele, myelomeningocele. Death
usually occurs by age 1 year.
Overlapping fingers
Patau syndrome Findings: severe intellectual disability, rocker- Incidence 1:15,000.
(trisomy 13) bottom feet, microphthalmia, microcephaly, Puberty (13).
cleft liP/Palate, holoProsencephaly, Defect in fusion of prechordal mesoderm
Polydactyly, cutis aPlasia, congenital heart Ž midline defects.
(Pump) disease, Polycystic kidney disease,
omphalocele. Death usually occurs by age 1.

Cutis aplasia

Nondisjunction in meiosis I Nondisjunction in meiosis II 1st trimester screening
Trisomy β-hCG PAPP-A
Meiosis I 21  
18  
Nondisjunction 13  

Meiosis II 2nd trimester screening
Trisomy β-hCG Inhibin A Estriol AFP
Nondisjunction
21    
18  — or   
Gametes 13 — — — —
n + 1 n + 1 n – 1 n – 1 n n n – 1 n + 1
Trisomy Monosomy Normal Monosomy Trisomy

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