Disorders caused by the inheritance of a single defective gene are known as monogenic diseases or single gene disorders. Monogenic diseases fall into two main categories. Firstly there are ‘recessive’ diseases, which do not produce any symptoms unless a defective copy of the gene is passed on by both the Mother and the Father.

The second category is comprised of disorders that are said to be ‘dominant’, which only require one defective copy of the gene to be inherited in order to occur.

Hundreds of different monogenic diseases, caused by errors in hundreds of different genes, have been discovered. Most of these disorders are very rare; however a few are relatively common. Well known monogenic diseases include cystic fibrosis, sickle cell anemia and Tay Sachs disease, which are recessive diseases, and myotonic dystrophy and Marfan syndrome, which are dominant.

Another category of monogenic diseases has an X-linked inheritance. Most couples at risk for an X-linked condition are identified by review of the family history or the birth of an affected child. X-linked conditions are caused by a change, or mutation, in a gene on the X chromosome and typically affect only males. This is because males have only one X chromosome, inherited from their mother, while they get a Y chromosome from their father. Since a male has only one X chromosome, if it has a mutated gene he will develop the disorder. It is this type of inheritance that causes disorders such as Duchenne Muscular Dystrophy, Hemophilia, Fragile X, etc.

Females are usually not affected because they have two X chromosomes. Females with one normal X chromosome and one mutated X chromosome generally do not have symptoms of the disease because of the presence of the normal gene. These females are referred to as “carriers” and are at risk for passing on the gene mutation to their children. If that child is a girl who inherits the gene, she also will be a carrier. Once the sex of the embryo is determined, female embryos, which would not be at risk for a sex-linked disorder, would be transferred.