X-linked (or sex-linked) inheritance refers to those (few) recessive genes that reside on the X chromosome. Females have two X chromosomes - they possess a chromosomal type of XX (males are XY, remember). If a girl possesses a defective trait on one of her X chromosomes, it will be suppressed1 by the activity of the dominant gene for the same trait on the other X chromosome. She will only exhibit2 the recessive condition if she has identical (defective) genes on both of her X chromosomes.

Males only possess one X chromosome - they are XY. This means that any defective gene on the X chromosome will automatically be expressed because it is unopposed by a normal gene on the other sex chromosome. For this reason some inherited diseases are almost exclusively diseases of males, and are said to be X-linked or sex-linked.

Examples of X-linked inheritance of diseases or conditions include hemophilia (blood clotting factor VIII or IX deficiency) or red-green color-blindness.

1. At least partially. Sometimes, the two genes produce partial disease. One gene makes the good enzyme; the other makes a defective enzyme. The measured level of good enzyme is then observed to be about 50% of normal. This may result in a normally functioning individual who can nevertheless be identified by this reduced enzyme level. This fact forms the basis for many genetic tests used to counsel patients and their families about who might carry a certain trait.

2. Technically, the gene will be said to be expressed.

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