X-linked recessive inheritance

Used to describe a characteristic or condition in which an alteration in one gene on the X chromosome causes the condition in males but not usually when females have one copy of the altered gene.  Males have only one X chromosome, the other sex chromosome being a Y chromosome;  females have two X chromosomes. 

Female-x-linked-inheritance-small-imageMale x-linked inheritance

X-linked recessive conditions

X-linked recessive conditions are caused by a gene alteration on the X chromosome.  As males have only one X chromosome, if they have a gene alteration on their X chromosome they will develop the condition.

Females rarely show signs of X-linked recessive conditions as they usually have a second unaltered copy of the gene on their other X chromosome to compensate for an altered gene (see X-inactivation).  A female who has a gene alteration on one of her X chromosomes is said to be a carrier for the X-linked recessive condition.

Males transmit their Y chromosome to their sons.  This means that sons will not inherit an X-linked recessive condition from their father.

x-linked-recessive-pedigree-example

Features of X-linked recessive inheritance that you may see on a family tree include:

  • males affected almost exclusively
  • the gene alteration can be transmitted from female carriers to sons
  • affected males cannot transmit the condition to their sons.

When the mother is a carrier for an X-linked recessive condition, each daughter has a 1 in 2 (50%) chance of inheriting the gene alteration and so being a carrier for the condition.  Each son has a 1 in 2 (50%) chance of inheriting the gene alteration and so inheriting the condition. 

When the father has an X-linked condition, his sons will not be affected.  His daughters will be a carrier for the condition.

Examples of X-linked recessive conditions include:

X-linked recessive inheritance - where the mother is a carrier for the condition

X-linked recessive inheritance - where the father has the condition

 

X-inactivation

In the very early female embryo, when only a few cells in size, one of the X chromosomes is switched off (inactivated) in every cell present.  This happens separately in each cell at random, but determines which X is "inactivated" in the female child or adult in all the descendant cells from that one cell.  Where female carriers of an X-linked recessive condition do manifest some symptoms, this can be due to a less random (skewed) X-inactivation process, or if they do not have a normal copy of that gene.

X-linked dominant

The inheritance pattern describing a dominant trait or condition caused by a mutation in a gene on the X chromosome.  The condition is expressed in heterozygous females as well as males, who have only one X chromosome.  Affected males tend to have more significant disease than affected females.  Disorders inherited in this manner are relatively rare.

Further resources

Visit our 'Explaining inheritance' section for a template to use in clinical practice, to help you explain autosomal and x-linked inheritance to patients. For chromosomal conditions you'll find worksheets showing pictures of real chromosomes down the microscope for males, females, Down, Patau, Edwards, Turner and Klinefelter syndrome. These can be used to help a patient understand the physical basis of a chromosomal condition in a family.

If you teach genetics, take a look through our presentations section for PowerPoint packages on a variety of topics including inheritance, conditions and family history.