Sometimes the condition with autosomal dominant inheritance may arise due to a mutation in egg or sperm, and in such cases there would be no preceding history. This is illustrated in the diagram below. In such cases the parents are unwitting carriers of the gene. Examples of such conditions are Friedreich's Ataxia , Cystic Fibrosis or Phenylketonuria . Unless the parents are related, the chances of marrying a carrier of the same recessive gene is low, though the incidence of the existence of recessive genes in the population varies with condition.
Genetic counselling can help to predict the occurrence for individual families.
This is a recessive form of inheritance where the mother carries the affected gene on the X chromosome. This means that girls are carriers and that usually only boys are affected by the disorder. Affected men will not pass the condition on to their sons but all their daughters will be carriers. This is because a man passes his Y chromosome on to his sons and his X chromosome to his daughters. There are few examples of this type of inheritance, one such is Coffin-Lowry Syndrome.