27th February 2024

X-linked dominant

The sex chromosomes ( X and Y chromosomes) are the primary chromosomes that determine the sex of the developing embryo. Males only possess one copy of the X chromosome, while female somatic cells undergo the process of X-inactivation, creating mosaicism for the X-chromosome activity in females.

The X-linked dominant condition presents similarly to an autosomal dominant pedigree due to the heterozygote genotype being sufficient to produce the phenotype. Due to the impact of mosaicism, females will typically present with a milder phenotype. Males usually have a more severe phenotype with only one X chromosome copy, and some conditions may result in a lethal phenotype.

Affected males will not pass on their genotype to their male children, while all female children will be affected. Affected females will have a 50% chance of passing the condition to both male and female children.

Image Source: Wikimedia Commons (2019). Drawing – X-Linked Dominant (1), Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Wiki_Drawing_-_X- Linked_Dominant_(1).svg (Accessed: 5 January 2024).