What is the notation for the female who is a carrier of an X-linked recessive disease?

What is the notation for the female who is a carrier of an X-linked recessive disease?

• A. XH XH (homozygous normal female)
• B. Xl XH (heterozygous female)
• C. Xl Xl (homozygous affected female)
• D. Xl Y (hemizygous male carrier)

Answer: (B)

The key idea is that a female carrier for an X-linked recessive disease is heterozygous: she has one chromosome with the disease allele and one normal chromosome. This is not the same as having two disease alleles or two normal alleles. The notation for that carrier state is Xl XH, with Xl representing the X chromosome carrying the recessive disease allele and XH representing the normal X chromosome. This explains why carriers typically don’t express the disease (the normal X can mask the recessive allele), though some carriers may show mild effects due to skewed X-inactivation. In contrast, Xl Xl would be two disease alleles and would be affected; XH XH would be completely normal; Xl Y would be a male with the disease (since males have only one X).

The key idea is that a female carrier for an X-linked recessive disease is heterozygous: she has one chromosome with the disease allele and one normal chromosome. This is not the same as having two disease alleles or two normal alleles. The notation for that carrier state is Xl XH, with Xl representing the X chromosome carrying the recessive disease allele and XH representing the normal X chromosome. This explains why carriers typically don’t express the disease (the normal X can mask the recessive allele), though some carriers may show mild effects due to skewed X-inactivation. In contrast, Xl Xl would be two disease alleles and would be affected; XH XH would be completely normal; Xl Y would be a male with the disease (since males have only one X).