<Disease H is a form of heritable eye cancer, resulting from the loss of functions for both alleles of gene H in chromosome X. One of the most commonly found mutations for the gene H is identified in Exon 5 of the gene, the coding sequence of which is given below:

5’ .. AAA GCG ACG GAT GAG AAG….3’ to

5’ .. AAA GCG ACG GAA TGA GAA….3’.

Mary suffers from this disorder but her mother and her younger brother are both perfectly free of the disease.

What is the mode of inheritance for the disease in Mary?

A. Autosomal, dominant
B. Autosomal, recessive
C. Sex-linked, recessive
D. Sex-linked, dominant
E. Not enough information on whether it is dominant or recessive>

I don't know the answer is C. Who can tell me? If it is recessive, the mother should be a carrier. But it said that she is perfectly free of the disease.
Thanks a lot!!!

the carrier is perfect free of the *disease*.

so it should be Sex(X)-linked, recessive.

She IS a carrier, but she is perfectly free of the disease. There is no contradiction between them. Because it's sex-linked recessive, Mary should have both alleles of gene H, meaning the father is sufferring from the cancer as well. Her brother is lucky because his X chromosome is from the normal allele of the mother and the Y chromosome(no gene H) from the father.

If the mode of inheritance is recessive, it means one allele of dysfunctional gene is not sufficient whereas dominant means one mutated gene can dominate the phenotype.

the disease is recessive, no symptom, quite right.