ABSTRACT
McKusick's Mendelian Inheritance in Man, accessed in its online version is the most complete single source for rare conditions. Mendelian inheritance may be established in several ways and the more independent evidence one has supporting the same conclusion, the more confident one can be that the risks one has given are correct. In some cases, the pattern of transmission of the disorder in the family may be conclusive, even if the diagnosis is unknown or proves to be erroneous. Late or variable onset of a disorder such as Huntington’s disease or adult polycystic kidney disease can be a major problem. In a small but important group of dominantly inherited disorders, individuals known to carry the relevant gene — obligate carriers, because they have an affected parent and affected offspring — may show no evidence of disease, even at an advanced age.
