Genetics Index Glossary

Aneuploidy and Deletions

Euploidy is the condition of having a normal number of structurally normal chromosomes. Euploid human females have 46 chromosomes (44 autosomes and two X chromosomes), and euploid bulls have 60 chromosomes (58 autosomes plus an X and a Y chromosome).

Aneuploidy is the condition of having less than or more than the normal diploid number of chromosomes, and is the most frequently observed type of cytogenetic abnormality. In other words, it is any deviation from euploidy, although many authors restrict use of this term to conditions in which only a small number of chromosomes are missing or added.

Generally, aneuploidy is recognized as a small deviation from euploidy for the simple reason that major deviations are rarely compatible with survival, and such individuals usually die prenatally.

The two most commonly observed forms of aneuploidy are monosomy and trisomy:

  • Monosomy is lack of one of a pair of chromosomes. An individual having only one chromosome 6 is said to have monosomy 6. A common monosomy seen in many species is X chromosome monosomy, also known as Turner's syndrome. Monosomy is most commonly lethal during prenatal development.

  • Trisomy is having three chromosomes of a particular type. A common autosomal trisomy in humans in Down syndrome, or trisomy 21, in which a person has three instead of the normal two chromosome 21s. Trisomy is a specific instance of polysomy, a more general term that indicates having more than two of any given chromosome.

Another type of aneuploidy is triploidy. A triploid individual has three of every chromosome, that is, three haploid sets of chromosomes. A triploid human would have 69 chromosomes (3 haploid sets of 23), a triploid dog 117 chromosomes. Production of triploids seems to be relatively common and can occur by, for example, fertilization by two sperm. However, birth of a live triploid is extraordinarily rare and such individuals are quite abnormal. The rare triploid that survives for more than a few hours after birth is almost certainly a mosaic, having a large proportion of diploid cells.

A chromosome deletion occurs when the chromosome breaks and a piece is lost. This of course involves loss of genetic information and results in what could be considered "partial monosomy" for that chromosome.

A related abnormality is a chromosome inversion. In this case, a break or breaks occur and that fragment of chromosome is inverted and rejoined rather than being lost. Inversions are thus rearrangements that do not involve loss of genetic material and, unless the breakpoints disrupt an important gene, individuals carrying inversions have a normal phenotype.

Next Topic for Cytogenetics: Translocations

Last updated on December 8, 1996
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