Sex-linked recessive

• Alternative Names

  Inheritance - sex-linked recessive; Genetics - sex-linked recessive; X-linked recessive

• Information

  X-linked diseases usually occur in males. Males have only one X chromosome. A single recessive gene on that X chromosome will cause the disease.

  The Y chromosome is the other half of the XY gene pair in the male. However, the Y chromosome doesn't contain most of the genes of the X chromosome. It therefore doesn't protect the male. This is seen in diseases such as hemophilia and Duchenne muscular dystrophy.

  TYPICAL SCENARIOS

  For a given birth, if the mother is a carrier (only one abnormal X chromosome) and the father is normal:

  • 25% chance of a normal boy
  • 25% chance of a boy with disease
  • 25% chance of a normal girl
  • 25% chance of a carrier girl without disease

  If the father has the disease and the mother is normal:

  • 100% chance of a normal boy
  • 100% chance of a carrier girl without disease

  X-LINKED RECESSIVE DISORDERS IN FEMALES

  Females can get an X-linked recessive disorder, but this is very rare. An abnormal gene on the X chromosome from each parent would be required, since a female has two X chromosomes. This could occur in the two scenarios below.

  For a given birth, if the mother is a carrier and the father has the disease:

  • 25% chance of a healthy boy
  • 25% chance of a boy with the disease
  • 25% chance of a carrier girl
  • 25% chance of a girl with the disease

  If the mother has the disease and the father has the disease:

  • 100% chance of the child having the disease, whether boy or girl.

  The odds of either of these two scenarios are so low that X-linked recessive diseases are sometimes referred to as “male only” diseases. However, this is not technically correct.

  Female carriers can have a normal X chromosome that is abnormally inactivated. This is called "skewed X-inactivation." These females may have symptoms similar to those of males.

Review Date: 5/15/2008

Reviewed By: Chad Haldeman-Englert, MD, Division of Human Genetics, Children's Hospital of Philadelphia, Philadelphia, PA. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.