What is the notation for a male with an X-linked recessive disease?

What is the notation for a male with an X-linked recessive disease?

• A. Xl Y.
• B. Xl Xl.
• C. XH Y.
• D. Xl XH.

Answer: (A)

In X-linked recessive inheritance, males express the disease whenever their single X chromosome carries the mutant allele, since they have only one X and nothing to mask it with. The notation Xl Y means the X chromosome has the recessive disease allele (l) and the Y is the male chromosome—so a male with Xl Y shows the disease. For contrast, a female would need two copies to be affected: Xl Xl. A female with one mutant and one normal allele would be a carrier: Xl XH. An unaffected male would have the normal X: XH Y.

In X-linked recessive inheritance, males express the disease whenever their single X chromosome carries the mutant allele, since they have only one X and nothing to mask it with. The notation Xl Y means the X chromosome has the recessive disease allele (l) and the Y is the male chromosome—so a male with Xl Y shows the disease.

For contrast, a female would need two copies to be affected: Xl Xl. A female with one mutant and one normal allele would be a carrier: Xl XH. An unaffected male would have the normal X: XH Y.