The random permanent inactivation of 1 of the 2 X chromosomes in the female, resulting in the lack of expression of the majority of genes on that chromosome, is a significant event during early development of the human embryo. The time of X-inactivation is not precisely known but is thought to vary over a period of several cell divisions during the blastocyst–gastrula transition. X-inactivation is also known as lyonization, after its discoverer, Mary Lyon. Lyonization affects the severity of the phenotype of several X-linked retinal conditions, such as RP and incontinentia pigmenti.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.