The basics of XLH genetics

Technically, XLH refers to hypophosphatemia that is "X-linked" (i.e., the relevant genetic defect is on the X chromosome), and the genetic transmission is explained below. Note, however, that the "autosomal dominant" version (where the relevant genetic defect is not on the X chromosome) is informally referred to as XLH, since the symptoms and treatment are the same, but the genetic transmission is slightly different. The only way to know for sure which version a patient has is through genetic testing. Whether the defect is on the X chromosome or elsewhere does not appear to affect the symptomatology, although there is no scientific research on the topic.

There's a good explanation of the genetic transmission of x-linked conditions (both dominant and recessive; keep in mind that XLH is a DOMINANT condition, while there are both dominant and recessive versions of the autosomal hypophosphatemias) here (and click on the illustrations if you're a visual learner): http://ghr.nlm.nih.gov/handbook/inheritance/inheritancepatterns

You can find information about autosomal transmission there as well. Again, it is believed that in MOST autosomal cases, the condition is DOMINANT, although a vanishingly small number of recessive cases have been identified.

The simple version (for true X-linked hypophosphatemia) is:

A woman has a 50/50 chance of transmitting the defective gene to each of her children, who will then have XLH. If she has four children, regardless of their gender, the odds are that two would have XLH and two would not. (Of course, that's just a statistical average, and actual outcomes may be different.)

A man will transmit the defective gene to ALL of his DAUGHTERS ONLY (and none of his sons). Thus, statistically, if he has four children, and two are girls and two are boys, then both girls will have XLH and neither boy will have XLH.

One frequently repeated myth is that someone can be a "carrier" for XLH without having the condition him/herself. That is only true of recessive conditions, and X-linked hypophosphatemia (including most autosomal versions) are DOMINANT. That means that if the person has the genetic defect, and is capable of passing it on to the next generation, he/she will also have hypophosphatemia.

This misconception arises from three common situations with XLH: 1. the symptoms are variable, so a mildly affected parent (especially women who tend to have less severe symptoms at least during childhood and early adulthood) may not be aware he/she has it until his/her child is diagnosed with XLH and doctors then check the parents' blood levels for phosphorus wasting. If the parent is never properly diagnosed, it may appear as if he/she was a carrier.  2. There can be some confusion even by genetic counselors, since most x-linked conditions are recessive, rather than dominant, so the counselor may assume that XLH is recessive, and therefore has carriers. 3. Finally, a substantial number of XLH patients are "spontaneous" cases. This just means that the defect first occurred in that patient's DNA during conception, much like any birth defect happens.

There's a natural, human tendency to want to explain away the spontaneous mutation, to look for someone in the family who might have had been a carrier. Unless that person was one of the patient's parents (see the first example of an undiagnosed case) who simply hadn't been diagnosed, there won't be an answer in the family tree. The mutation is simply a random event. Neither parent has XLH and neither parent is a carrier. The child simply has a new, or "spontaneous" mutation, and the children of the spontaneous XLH patient will inherit in the pattern described above.