Genetic Diseases

Sex linked genetic diseases include various muscular dystrophies such as Duchenne muscular dystrophy, hemophilia, Charcot-Marie-Tooth disease and color blindness. They are called sex linked diseases because there is an abnormal gene that is carried on the X chromosome.
 
A female is made up of two X chromosomes (one from her father and one from her mother).  If she inherits one defective X chromosome, she will still be normal as long the second X chromosome is normal.  Therefore it is very rare for a female to have a sex linked disease because the normal X chromosome balances out the abnormal X chromosome.  If she has one defective X chromosome, instead of having the disease, she will be a carrier of this disease and possibly transmit it to her sons.

A male is made up of an X chromosome from his mother, and a Y chromosome from his father.  If he inherits a defective X chromosome, he will have the disease because his abnormal X chromosome cannot be balanced by his normal Y chromosome.  Diseases linked to the Y chromosome are extremely rare. 

The best way to prevent transmission of a sex linked disease is to undergo in vitro fertilization with preimplantation genetic diagnosis.  IVF/PGD allows the embryologist to screen the embryo for defects before it is transferred to the mother and a pregnancy occurs.

Autosomal Linked Genetic Diseases

In each normal human cell there are 23 pairs of chromosomes, one pair is the sex chromosome, and the remainder are called the autosomal chromosomes.   These define a persons individual human traits.  Autosomal linked genetic diseases consists of either additions or deletions of any pair of autosomal chromosomes.

Using fluorescent in situ hybridization (FISH) (please see the PGD section of the Web site) the embryologist is able to count the number of chromosomes commonly involved in syndromes such as Chromosome 21 or Chromosome 18.  If an embryo shows any of these defects, it is not used for transfer to the uterus.

Single Gene Defects

Some disorders result when a mutation causes a single gene to be damaged or missing. Examples of this kind of disorder are Sickle-Cell Anemia, Tay-Sachs Disease, Thallasemia, Cystic Fibrosis and Down's Syndrome.