Not only can the sex of the parent from whom a gene is inherited determine whether it is turned "on" or turned "off," but the sex of the parent may also influence whether certain abnormal sections of chromosomes become more abnormal. For example, the sex of the parent contributing the X chromosome may increase or decrease the chance that a child will be affected with fragile X syndrome.
Fragile X syndrome occurs in one out of 1,000 male births and one out of 2,000 female births. Males are affected more severely than females and the syndrome may be more pronounced if the child inherits the disorder from his/her mother. Part of this is explained by the fact that fragile X syndrome is caused by an abnormality of the X chromosome. Remember that a male is XY and a female is XX. A male child receives a Y chromosome from the father and an X chromosome from the mother. A female child, however, can receive an X from either the mother or the father. Girls with fragile X syndrome are less severely affected than boys because they have a normal X chromosome that helps to protect them from the abnormal X chromosome. However, it was somewhat perplexing that girls were affected at all.
This mystery was solved when researchers learned that there is a range of abnormality in the fragile X chromosome. If the abnormality of the fragile X region of the chromosome is severe, the influence can be strong enough to affect females. If the abnormality is mild, females will not have symptoms of fragile X syndrome. Furthermore, the fragile X region of the X chromosome may become more severe when it is maternally inherited. The sex of the parent that the region is inherited from affects whether the chromosome abnormality remains stable or becomes greater.
Many other conditions are associated with similar chromosome abnormalities and may remain stable or become more severe depending upon whether the chromosome region is inherited from the mother or the father. In some of these conditions, the region becomes more abnormal when it is paternally inherited. Huntington disease, an adult onset neurological disease, is one such condition.