Lyon hypothesis

synonyms: Lyon law, lyonization, X-inactivation
https://doi.org/10.1351/goldbook.10906

Hypothesis (named after Mary Lyon) that random inactivation of all but one X chromosome occurs in each female mammalian cell in early development, thus explaining why the effect of the X chromosome on phenotype is the same in males with one copy as in females with two copies.

Notes:
  1. The Barr body, visible in some female cells, is an inactivated X chromosome.
  2. This phenomenon leads to mosaicism for X-linked genes in the female, since the paternal X chromosome is inactivated in some cells and the maternal one in others.
  3. The Lyon hypothesis was designated the Lyon law by the European Molecular Biology Organization (EMBO) in 2011.
Source:
PAC, 2016, 88, 713. (Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)) on page 774 [Terms] [Paper]
Citation: 'Lyon hypothesis' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.10906 RIS BibTex EndNote
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