Question

a female patient of a genetic counselor has a brother with a sex - linked recessive disease carried on the x chromosome. her grandfather on her mothers side also had the disease. the patient wants to know the probability that she is a carrier of the disease.
what does the counselor know about the patients chance of being a carrier?
the patient is definitely not a carrier because her father does not have the disease.
there is a 1 out of 2 chance that the patient is a carrier because her father is definitely a carrier.
there is a 1 out of 2 chance that the patient is a carrier because her mother is definitely a carrier.
the patient is definitely a carrier because both of her parents must be carriers since the patients brother has the disease.

Explanation:

Since the disease is sex - linked recessive and carried on the X chromosome. The patient's brother has the disease. Let's assume the mother is \(X^AX^a\) (carrier) and the father is \(X^AY\) (normal). The female patient gets one X chromosome from her father (\(X^A\)) and one from her mother. The mother has a 50% chance (\(1\) out of \(2\)) of passing the \(X^a\) (carrier allele) to the daughter.

Answer:

There is a 1 out of 2 chance that the patient is a carrier because her mother is definitely a carrier.