Errors in meiosis result in gametes with abnormal numbers of chromosomes and production, after fertilization, of an aneuploid conceptus. The genetic imbalance resulting from the loss of an entire chromosome in individuals with deletion‚Äźtype aneuploidy is generally lethal, except for the full X chromosome monosomy. Most survivable partial monosomies are associated with significant dysmorphism and mental retardation.

Keywords: full monosomy; partial monosomy; microdeletion syndrome; Turner syndrome; nondisjunction

Figure 1.

Example of the mechanism of chromosomal nondisjunction: (A) normal meiosis; (B) meiosis with chromosomal nondisjunction visible in II. I: metaphase I; II: telophase I–metaphase II showing nondisjunction in (B) (left cell); III: telophase II showing in (B), for chromosome 19, two disomic and two nullisomic daughter cells.

Figure 2.

Example of the mechanism of nondisjunction of chromatids: (A) normal meiosis; (B) meiosis with nondisjunction of chromatids visible in III. I: metaphase I; II: telophase I–metaphase II with normal chromosomal disjunction in A and B; III: telophase II showing in B one disomic daughter cell resulting from nondisjunction of the sister chromatids of chromosome 19, one subsequent nullisomic cell and two normal cells.

Figure 3.

Expected segregation of the chromosomes within a quadrivalent in a 2:2 model. Each daughter cell receives two of the chromosomes of the quadrivalent in three ways: 1 and 2, adjacent I (1) and adjacent II (2) segregation – two chromosomes with adjacent centromeres migrate together yielding chromosomal imbalance; 3, alternating segregation – two chromosomes with alternate centromeres migrate together yielding normal or chromosomally balanced daughter cells. Other models (3:1 or 4:1 segregation) can occur, but they appear to be much rarer.



Angell RR (1997) First‐meiotic‐division nondisjunction in human oocytes. American Journal of Human Genetics 61: 23–32.

Clement‐Jones M, Schiller S, Rao E, et al. (2000) The short stature homeobox gene SHOX is involved in skeletal abnormalities in Turner syndrome. Human Molecular Genetics 9: 695–702.

Dailey T, Dale B, Cohen J and Munne S (1996) Association between nondisjunction and maternal age in meiosis‐II human oöcytes. American Journal of Human Genetics 59: 176–184.

Donnelly SL, Wolpert CM, Menold MM, et al. (2000) Female with autistic disorder and monosomy X (Turner syndrome): parent‐of‐origin effect of the X chromosome. American Journal of Medical Genetics 96: 312–316.

Guttenbach M, Koschorz B, Bernthaler U, Grimm T and Schmid M (1995) Sex chromosome loss and aging: in situ hybridization studies on human interphase nuclei. American Journal of Human Genetics 57: 1143–1150.

Hook EB (1983) Chromosome abnormalities and spontaneous fetal death following amniocentesis: further data and association with maternal age. American Journal of Human Genetics 35: 110–116.

Powell CM (1999) Sex chromosomes and sex chromosome abnormalities. In: Gersen SL and Keagle MB (eds.) The Principles of Clinical Cytogenetics, pp. 229–258. Totowa, NJ: Humana Press.

Further Reading

Claeys I, Holvoet M, Eyskens B, et al. (1997) A recognisable behavioural phenotype associated with terminal deletions of the short arm of chromosome 8. American Journal of Medical Genetics 74: 515–520.

Devriendt K, Matthijs G, Van Dael R, et al. (1999) Delineation of the critical deletion region for congenital heart defects, on chromosome 8p23.1. American Journal of Human Genetics 64: 1119–1126.

Hsu YF (1994) Phenotype/karyotype correlation of Y chromosome aneuploidy with emphasis on structural aberrations in postnatally diagnosed cases. American Journal of Medical Genetics 53: 108–140.

Lukusa T, Devriendt K and Fryns JP (1999) A 3p deletion syndrome in a child with both del(3)(p25→pter) and dup(17)(q23→qter). Annales de Génétique 42: 91–94.

Web Links

Short stature homeobox (SHOX); Locus ID: 6473. LocusLink:

Short stature homeobox (SHOX); MIM number 312865. OMIM:‐post/Omim/dispmim?312865

Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

How to Cite close
Fryns, Jean‐Pierre, and Lukusa, Tshilobo Prosper(Jan 2006) Monosomies. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0005545]