Why is m'athair not mo mhathair?

In Irish, "m'athair" means "my father," while "mo mhathair" means "my mother." The reason for the difference lies in the initial mutation of the Word "máthair" (mother), which undergoes lenition (softening) due to the preceding possessive pronoun "mo" (my). In contrast, "athair" (father) does not experience this change, hence the form "m'athair" is used instead of "mo athair."