Sex or X-linked means the gene is carried on the X chromosome. So the pattern of inheritance and disease is somewhat different than for autosomal (not-sex chromosome) genes. X-linked traits are commonly seen only in males, who got it from their mother (dad gave them a Y-chromosome to make them boys). Likely half their sisters are carriers of the trait just like mom was. And you cannot tell which sister is clear & which a carrier until they have pups. Some forms of cryptorchidism (retained testicles) as well as some bleeding disorders are thought to be so inherited. DCM (cardiomyopathy) in Danes looks like it might be inherited as an X-linked, or perhaps a mitochrondrial, trait. You don't blame the daddy for this, but rather acknowledge that momma, unseen, is the author of the trait in question. (Some X-linked traits need another gene for disease to express, like epilepsy in Standard Schnauzers: in this case both parents *may* be involved in the inheritance pattern; this may well be the case for "cardio" as well. See "Polygenetic" below.) And don't forget the sisters of any such affected brother are likely carriers as they have one X gene from their momma, so pedigrees should be properly marked to reflect this, even though these sisters may never be unwell. There is another peculiar sort of inheritance that is purely from the mother called mitochondrial DNA inheritance. These are the little dynamos that give cells energy & mammals get all their mitochrondria from their mommas. Mitochondrial DNA is thought to be behind some forms of cardiomyopathy & in these cases, again, the mother is "at fault", not the dad, although both sons & daughters would more likely be equally affected.