The methemoglobinemia condition experienced in the Fugate family is rare but has been reported in other individuals. It is a recessive trait and may appear when two people who carry mutations on CYB5R3 gene (22q13.31-qter). This particular mutation causes a form of methemoglobinemia known as RCM Type 1, which results in a blue skin tone (cyanosis) at birth and has few health effects. RCM Type 1 often will cause shortness of breath on exertion. There is a second type of methemoglobinemia which is far more severe (RCM Type 2). Type 2 individuals have microcephaly and developmental delay in addition to the blue skin tone. Type 2 is caused by a total loss of NADH-cytochrome b5 reductase (Cb5R) function due to mutations on the CYB5R3 gene.
In short, there have been other instances of people who have methemoglobinemia throughout history. It is a rare condition and was more frequent in the Appalachians since the population was isolated and cousins married each other, amplifying the prevalence of the genetic mutations in that community.