<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!!!