Sex-Linked Recessive

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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