Merck Manual

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X-Linked Recessive Disorders
X-Linked Recessive Disorders

If a gene is X-linked, it is present on the X chromosome. Recessive X-linked disorders usually develop only in males. This male-only development occurs because males have only one X chromosome, so there is no paired gene to offset the effect of the abnormal gene. Females have two X chromosomes, so they usually receive a normal or offsetting gene on the second X chromosome. The normal or offsetting gene normally prevents females from developing the disorder (unless the offsetting gene is inactivated or lost).

If the father has the abnormal X-linked gene (and thus the disorder) and the mother has two normal genes, all of their daughters receive one abnormal gene and one normal gene, making them carriers. None of their sons receive the abnormal gene because they receive the father’s Y chromosome.

If the mother is a carrier and the father has normal genes, any son has a 50% chance of receiving the abnormal gene from the mother (and developing the disorder). Any daughter has a 50% chance of receiving one abnormal gene and one normal gene (becoming a carrier) and a 50% chance of receiving two normal genes.