But what happens if they are related to each other? Consider the following: A parent transmits to their offspring one of the two copies they have of each gene. If one of the parents is a carrier, then the chances are 1/2 that their children will also be carriers. Given that cystic fibrosis is a relatively common genetic disease, it works out that the chances that two siblings will both be carriers of cystic fibrosis is 1/50, as compared to 1/625 for two unrelated persons. For first cousins, the risk is 1/200 and for second cousins, the risk is the same as if they were unrelated. PKU is a rarer genetic disease that if untreated, will cause severe mental retardation. The occurrence is 1/12000 live births. The chances that two unrelated people are carriers is 1/2500. For two siblings, the risk 1/100, for first cousins, it is 1/400 and for second cousins, it is 1/1600. Thus, the real risk is with brother-sister marriages, and to a lesser extent, first cousin marriages. Any relations beyond these are essentially at the same risk as the general population.