Alternative names
Inheritance - sex-linked dominant; Sex-linked dominant; X-linked dominant

Definition

The term “sex-linked dominant” means that a single abnormal gene on the X chromosome can cause the disease. The disease is likely to be transmitted to boys and girls. However, boys may not survive. This is a rare mode of transmission.

Related terms and topics:

 
• Gene  
• Chromosome  
• Inheritance  
• Heredity and disease  
• Genetic counseling and prenatal diagnosis  
• Sex-linked recessive  
• Autosomal dominant  
• Autosomal recessive

Information

The inheritance of genetic diseases, abnormalities, or traits is described by both the type of chromosome the abnormal gene resides on (autosomal or sex chromosome), and by whether the gene itself is dominant or recessive.

Autosomally inherited diseases are inherited through the non-sex chromosomes, pairs 1 through 22. Sex-linked diseases are inherited through one of the “sex chromosomes” (the X chromosome or the Y chromosome).

Dominant inheritance occurs when an abnormal gene from ONE parent is capable of causing disease even though there may be a matching gene from the other parent that is normal. The abnormal gene dominates the outcome of the gene pair.

Recessive inheritance occurs when BOTH matching genes must be abnormal to produce disease. If only one gene in the pair is abnormal, the disease is not manifest or is only mildly manifest. However, the genetic predisposition to disease can be passed on to the children.

CHANCES OF INHERITING A TRAIT

For an X-linked dominant disorder: If the father carries the abnormal X gene, all of his daughters will inherit the disease and none of the sons will have the disease. If the mother carries the abnormal X gene half of all their children (daughters and sons) will inherit the disease tendency.

In other words, if it is assumed that 4 children are produced (2 male and 2 female), the mother is a carrier (1 abnormal X, she has disease), and the father is normal, the STATISTICAL expectation is for:

 
• 2 children (1 girl and 1 boy) with disease  
• 2 children (1 girl and 1 boy) normal

If it is assumed that 4 children are produced (2 male and 2 female), the father is a carrier (abnormal X, he has disease), and the mother is normal, the STATISTICAL expectation is for:

 
• 2 girls with disease  
• 2 normal boys

This does not mean that children WILL necessarily be affected.

Medical Encyclopedia

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