Aneuploidy is defined as an abnormal number of chromosomes. Syndromes caused by an extra or missing chromosome are among the most widely recognized genetic disorders in humans. Different organisms have widely varying chromosome complements and thus the term "aneuploidy" does not refer to a particular number of chromosomes, but rather the situation in which the chromosome content within a given cell is abnormal.

Chromosome abnormalities are detected in 1 of 160 live human births. Apart from sex chromosome disorders, most cases of aneuploidy result in death of the developing fetus (miscarriage); the most common extra autosomal chromosomes among live births are 21, 18 and 13.


  • Dysmorphic face
  • Mental retardation
  • Concentration difficulties
  • Learning difficulties
  • Microcephaly


Germline aneuploidy is typically detected through karyotyping, a process in which a sample of cells is fixed and stained to create the typical light and dark chromosomal banding pattern and a picture of the chromosomes is analyzed.

Other techniques include Fluorescence In Situ Hybridization (FISH), Quantitative Polymerase Chain Reaction (PCR) of Short Tandem Repeats, Quantitative Fluorescence PCR (QF-PCR), Quantitative Real-time PCR (RT-PCR) dosage analysis, Quantitative Mass Spectrometry of Single Nucleotide Polymorphisms, and Comparative Genomic Hybridization (CGH). These tests can also be performed prenatally to detect aneuploidy in a pregnancy, either through amniocentesis or chorionic villus sampling.

Pregnant women of 35 years or older are offered prenatal diagnosis because the chance of chromosomal aneuploidy increases as the mother's age increases. For more information, see prenatal diagnosis.