Genetics Northern California

X-linked Recessive

X-linked recessive inheritance leads to a condition that is expressed by all males who have the altered gene on their X chromosome.

In X-linked conditions, the involved gene is on the X chromosome. Genes give the body the instructions it needs for growth and healthy development. They come in pairs and a baby receives one copy of each gene from the mother and one from the father. Chromosomes are the structures that house all of the genes. Each person has 23 pairs of chromosomes and each chromosome contains thousands of genes. The last pair is named with letters (X and Y). These two determine a person's sex (XX = female; XY = male). In a recessive condition, if a person has one working copy of a gene, it protects against the disease. However, if there are no working copies, the person will have the genetic condition.

Women who have an altered gene on one of their two X chromosomes also have an unaltered gene on their other X chromosome.  Women usually do not express any symptoms of an X-linked recessive disease because they have one working copy of the gene; it is said that they "carry" the condition.  The altered gene has a 50% chance to be passed by a woman to any of her sons and daughters. Sons who inherit the gene will be affected; daughters will be carriers.

Since males have only one X chromosome, if they have an altered gene on the X chromosome, they will be affected with the specific disorder (i.e. hemophilia, colorblindness). A man who has an X-linked recessive condition passes his X chromosome with the altered gene to all of his daughters, who will then be carriers of the condition. His sons inherit the Y chromosome instead of the X chromosome, so none of his sons will inherited the condition from him.