To answer this question, we have to consider that a sex-linked trait (in this case, disease) is a condition related to an allele of the sexual X chromosome. In females, we know that the inherited sexual chromosomes are XX, while in males, the inherited sexual chromosomes are XY. This leads to consider that for a given disease to be expressed in an individual, it depends on if they are male or female.
For example, if a male inherits an allele that carries the trait for the diseases, usually a recessive allele found in the X chromosome, in all cases they would express the disease. But if a female inherits only one allele for that trait, since they inherit two X chromosomes, they would not express the disease.
Considering this, if methemoglobinemia is expressed in one parent, for example, the male parent, and not in the female, we have then the following:
If the disease was sex-linked, only would be expressed when the parents have a boy, instead of a girl, and if that boy inherits only the allele of the disease, in this case, expressed with a lowercase m.
If the disease was not sex-linked, no matter whether the offspring would be males or females, there would be a rate of 25 % of they of having the disease.