X-chromosomal male letale mutation

X-linked mutations can be lethal or non-lethal for male embryos.

In the case of X-linked lethal mutations, only female embryos can survive, as they can build up a functional mosaic via the so-called Lyon effect.

This phenotype is observed almost exclusively in female individuals. Male embryos that have only one X chromosome die in utero. Female individuals survive because they can build a functional mosaic via the Lyon effect of X inactivation.

Examples of such cutaneous mosaics, which mostly manifest in the pattern of Blaschko lines, are incontinentia pigmenti, X-linked dominant chondrodysplasia punctata (Conradi-Hünermann-Happle syndrome), and focal dermal hypoplasia.

1. Happle R (2016) The categories of cutaneous mosaicism: a proposed classification. Am J Med Genet A 170A: 452-459
2. Moog U et al (2020) Diseases caused by genetic mosaicism. Dtsch Ärztebl Int 117: 119-125