Context: This passage describes the myotonia congenita trait in goats:

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

B