Context: This passage describes the Thomsen disease trait in humans:

In a group of humans, some individuals have Thomsen disease and others do not. In this group, the gene for the Thomsen disease trait has two alleles. The allele M is for having Thomsen disease, and the allele m is for not having Thomsen disease.
Tim, a human from this group, has Thomsen disease. Tim has one allele for having Thomsen disease and one allele for not having Thomsen disease.
Based on this information, what is Tim's genotype for the Thomsen disease gene?
A. Mm
B. having Thomsen disease
Pick the correct answer from the listed options and provide the letter of the selected option.

A