- Clinical science
Structural and numerical chromosomal aberrations may affect either the autosomes or gonosomes and are a common cause of spontaneous abortions. Autosomal aberrations that are frequently observed are trisomy 13 (Patau syndrome), trisomy 18 (Edwards syndrome), and the most common and widely recognized chromosomal aberration, trisomy 21 (Down syndrome). These conditions have an extra copy of the chromosome to which their name refer. The risk of autosomal aberrations increases with maternal age. Characteristic features include facial and skeletal malformations, which are usually recognizable at birth. The conditions are further associated with congenital heart defects and malformations of other internal organs. Turner syndrome and Klinefelter syndrome are gonosomal aberrations in which individuals have a missing X chromosome or an additional X chromosome, respectively. These conditions are primarily characterized by impaired development of secondary sexual characteristics and infertility secondary to gonadal dysgenesis. The diagnosis for all chromosomal aberrations is confirmed via karyotyping.
- Chromosomal aberrations: deviation from the normal chromosome constellation
- Triploidy: : presence of three sets of chromosomes
- Trisomy: : presence of triplicate instead of duplicate number of a particular chromosome or part of a chromosome
Chromosomal aberrations are the most common cause of spontaneous abortions (accounting for 60% of cases).
- Approx. 50% of anomalies are trisomies.
- Approx. 20% of anomalies are triploidies.
Overview of viable
- Karyotype: : ♀: 47 XX+13; ♂: 47 XY+13
- Incidence: ∼ 1:7400 live births 
- Meiotic nondisjunction
- Abnormal fusion of prechordal mesoderm → midline defects
- Microcephaly, holoprosencephaly
- Characteristic facial anomalies
- Polydactyly, primarily hexadactyly, flexed fingers
- Congenital heart defects (particularly ventricular septal defect, patent ductus arteriosis)
- Rocker-bottom feet
- Visceral and genital anomalies, especially of the kidneys and ureters (e.g., polycystic kidney disease)
- Aplasia cutis congenita; : congenital absence of skin; most commonly scalp lesions with a punched-out appearance that may extend to the bone or the dura
- Capillary hemangioma
- Prognosis: Infants usually die before the age of 1, only approx. 11% of infants survive past 12 months of age. 
To remember that Patau syndrome is caused by trisomy 13, think of the age of Puberty onset (13).
To remember the most common features of this syndrome, think of the 7 Ps of Patau syndrome: holoProsencephaly, cleft liP and Palate, Polydactyly, Pump disease (congenital heart disease), Polycystic kidney disease, cutis aPlasia.
- Karyotype: ♀: 47 XX+18; ♂: 47 XY+18
- Pathogenesis: meiotic nondisjunction
- Gender: ♀ > ♂
- Characteristic facial anomalies
- Clenched fists with flexion contractures and overlapping fingers
- Rocker-bottom feet; : convex deformity of the plantar side of the foot; , with a vertical talus, and prominent calcaneus
- Congenital heart defects (particularly VSD, ASD, tetralogy of Fallot)
- Malformations of internal organs: diaphragmatic hernia, ureter, and kidneys (horseshoe kidneys)
- Severe intellectual disability
- Prognosis: Only approx.13% of patients survive past 12 months of age. 
To remember the most important features of Edwards syndrome, think “PRINCE Edward turned 18:” Prominent occiput, Rocker-bottom feet, Intellectual disability, Nondisjunction (in meiosis), Clenched fists, low-set Ears, and chromosome 18.
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