Phenotypic Ratio in Genetic Cross with a Metabolic Disorder

A genetic researcher investigates the inheritance patterns of a newly discovered gene responsible for a rare metabolic disorder, characterized by a mutation that impairs the function of a key enzyme. In a population study, they identify several carriers of the mutated allele (denoted as 'm') and the wild-type allele (denoted as 'M'). The disorder is expressed only in individuals who are homozygous for the mutated allele (mm). The researcher conducts a cross between two individuals, one homozygous dominant (MM) and another heterozygous (Mm), and observes the offspring in terms of genotype. What is the expected phenotypic ratio of the offspring from this cross?