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: - The Barr body, visible in some female cells, is an inactivated X chromosome.
- 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.
- 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 (https://doi.org/10.1515/pac-2015-1202)