An abnormal gene on the X chromosome from each parent is required to cause the disease in females since the female has 2 X chromosomes. In males there is only one X chromosome, therefore, a single recessive gene on the X chromosome will cause the disease.

Note: Although the Y chromosome is the other half of the XY gene pair in the male, the Y chromosome doesn't contain most of the genes of the X chromosome and therefore doesn't protect the male. Therefore, recessive genes on the X chromosome of the male will be expressed. This is seen in diseases such as hemophilia and Duchenne muscular dystrophy.

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

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

Dominant inheritance occurs when an abnormal gene from ONE parent is capable of causing disease even though the matching gene from the other parent 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.

STATISTICAL CHANCES OF INHERITING A TRAIT:
For an X-linked recessive disorder:

• If only the mother carries the gene and the father is normal, all of the female children will be normal (50% with 2 normal chromosomes and 50% carriers), one half of all the male children will exhibit the disease, and one half will be normal. The recessive gene is expressed in the male because there is not another X to counteract it, only the Y (which determines for maleness).
• If only the father caries the recessive gene, all of his daughters will be carriers and all of his sons will be normal.
• If both the mother carries the abnormal gene and the father has the disease, then STATISTICALLY out of 4 children 1 daughter will have the disease (two recessive genes on the X chromosome), 1 daughter will be a carrier, 1 son will have the disease (one recessive gene on the X and a Y chromosome) and the other son will be normal. In other words, 50% of the children (boys and girls) will be affected and 50% normal.

• 1 boy normal
• 1 boy with disease
• 1 girl normal
• 1 girl carrier without disease

• 2 boys normal
• 2 girls carriers without disease

• 1 girl with disease
• 1 girl carrier without disease
• 1 boy (abnormal X) with disease
• 1 boy normal