Chromosomal Deletions and Abnormalities

Deletion and Structural Abnormalities

• Deletion results in monosomy, usually incompatible with survival to term
• There are two types of deletions: terminal deletion and interstitial deletion

Terminal Deletion

• Involves a single break of the terminal part of a chromosome, resulting in loss of the broken part
• Example: Cri-du Chat syndrome (46,XX,5p- or 46,XY,5p-)
• Characteristics of Cri-du Chat syndrome:
  • Deletion of the short arm of chromosome 5
  • Incidence: 1 per 20,000 to 1 per 50,000 births
  • Underdevelopment of larynx, resulting in a characteristic cat-like cry (Mewing cry)
  • Mental retardation
  • Microcephaly and facial anomalies

Interstitial Deletion

• Involves two breaks, resulting in loss of the intervening portion of the chromosome
• Some interstitial deletions can occur as micro-deletions, which cannot be detected by karyotype and require FISH technique for detection
• Examples: Prader-Willi syndrome (interstitial deletion of paternal chromosome 15, 15q11–q13) and Angelman syndrome (interstitial deletion of maternal chromosome 15, 15q11–q13)

Other Structural Abnormalities

• Duplication: extra copies of a chromosome part
• Inversion: involves two breaks along the chromosome, resulting in reversal of a given segment
  • Pericentric inversion: inverted segment involves both p and q arms with centromere
  • Paracentric inversion: inverted segment involves only one arm (p or q)