Buy the Book PDA Download

Update Me E-mail alerts The Merck Manual Minute

Print This Topic Email This Topic

Chromosomal anomalies cause various disorders. Anomalies that affect autosomes (the 22 paired chromosomes that are alike in males and females) are more common than those that affect sex chromosomes (X and Y). Chromosomal abnormalities fit into several major categories:

• Trisomies (extra chromosomes)
• Translocations (anomalies in which segments of chromosomes inappropriately join with other chromosomes)
• Deletions and duplications of various chromosomes or parts of chromosomes
• Mosaicisms (anomalies in which a person starting from a single fertilized egg develops ≥ 2 cell lines differing in genotype [genetic constitution]).

Some specific terms from the field of genetics are important for describing chromosomal anomalies:

• Karyotype: The full set of chromosomes in a person's cells.
• Genotype: The genetic constitution determined by the karyotype.
• Phenotype: The person's outward appearance—the biochemical, physiologic, and physical makeup as determined by the genotype and environmental factors (see General Principles of Medical Genetics).

Diagnosis

Lymphocytes are used for chromosomal analysis, except prenatally, when amniocytes or cells from placental chorionic villi are used (see Prenatal Genetic Counseling and Evaluation: Amniocentesis). A karyotype analysis involves blocking cells in mitosis during metaphase and staining the condensed chromosomes. Chromosomes from single cells are photographed, and their images are arranged, forming a karyotype.

Several techniques are used to better delineate the chromosomes:

• In classical banding (eg, G [Giemsa]-, Q [fluorescent]-, and C-banding), a dye is used to stain bands on the chromosomes.
• High-resolution chromosome analysis uses special culture methods to obtain a high percentage of prophase and prometaphase spreads. The chromosomes are less condensed than in routine metaphase analysis, and the number of identifiable bands is expanded, allowing a more sensitive karyotype analysis.
• Spectral karyotyping analysis (also called chromosome painting) uses chromosome-specific multicolor fluorescent in situ hybridization (FISH) techniques that improve the visibility of certain defects, including translocations and inversions.
• Comparative genomic hybridization is a single-step technique that allows the entire genome to be scanned for mutations, including increases (duplications) or decreases (deletions) in DNA and unbalanced translocations.

Last full review/revision December 2008 by Gregory S. Liptak, MD, MPH

Content last modified December 2008