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 typically 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 2 normal genes, all of their daughters receive 1 abnormal gene and 1 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 the normal gene, 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 1 abnormal gene and 1 normal gene (becoming a carrier) and a 50% chance of receiving 2 normal genes.