X-linked recessive inheritance

X-linked recessive inheritance is a mode of inheritance in which a mutation in a gene on the X chromosome causes the phenotype to be always expressed in males (who are necessarily hemizygous for the gene mutation because they have one X and one Y chromosome) and in females who are homozygous for the gene mutation (see zygosity). Females with one copy of the mutated gene are carriers.

X-linked inheritance means that the gene causing the trait or the disorder is located on the X chromosome. Females have two X chromosomes while males have one X and one Y chromosome. Expression of X-linked conditions in female carriers can vary greatly due to random X-chromosome inactivation (Lyonization) within each cell. Differences in X-chromosome inactivation (known as skewed X-inactivation) occurs due to some cells expressing one X allele and some the other. Decades of research has led to the consensus that conditions be considered X-linked and that the "dominant" and "recessive" designations be retired.

The current estimate of sequenced X-linked genes is 499, and the total, including vaguely defined traits, is 983.

In humans, inheritance of X-linked recessive traits follows a unique pattern made up of three points.

A few scholars have suggested discontinuing the use of the terms dominant and recessive when referring to X-linked inheritance. The possession of two X chromosomes in females leads to dosage issues which are alleviated by X-inactivation. Stating that the highly variable penetrance of X-linked traits in females as a result of mechanisms such as skewed X-inactivation or somatic mosaicism is difficult to reconcile with standard definitions of dominance and recessiveness, scholars have suggested referring to traits on the X chromosome simply as X-linked.

The most common X-linked recessive disorders are:

Theoretically, a mutation in any of the genes on chromosome X may cause disease, but below are some notable ones, with short description of symptoms:

[Female X-linked disorders]