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.
History and Experience