Single gene defects are caused by single gene abnormalities. This is called a mutation. The mutation may be present on a single chromosome of a pair or on both chromosomes of the pair. In some cases the mutation can be in mitochondrial DNA. Mutations cause an error in the genetic information of the gene that alters the normal function of a cell. Gene mutations can alter the cells function due to a lack of a required protein. Single gene disorders usually show a characteristic family history of a specific genetic disease. Examples include Cystic fibrosis, Sickle cell disease, Fragile X, Thalassemia, and Duchenne muscular dystrophy.
Family History
PGD is recommended for families with a history of a specific genetic disease. Using polymerase chain reaction, fluorescent PCR and DNA sequencing, the scientists in our PGD laboratory can examine each developing embryo to identify the absence or presence of these specific genetic disorders. As a result, only those embryos free of genetic disease will be transferred to the patient’s uterus so as to increase the chance of conception and ultimately a healthy baby.
Single gene disorders are categorized depending upon whether the gene is located on the X chromosome, an autosome or whether the gene is dominant or recessive. These classifications include autosomal recessive, autosomal dominant and X-linked.
Dominant Disorders
For a dominant disorder, one only needs to have the abnormal DNA sequence on one chromosome. If that mutation is passed on to the embryo, the embryo will be affected with that genetic disease. One example of an autosomal dominant disorder is Myotonic dystrophy.
Recessive Disorders
Recessive disorders require that the mutation be present on both chromosomes of the chromosome pair. If one only has the mutation on one chromosome, the individual is normal but carries the mutation in his cells and is called a carrier. The fertilization of an egg from carrier parents may result in an embryo having the mutation on both chromosomes of the chromosome pair and the embryo therefore being affected with that genetic disease. For example, Cystic fibrosis (CF) is a common autosomal recessive genetic disorder that primarily affects the lungs of CF patients. The CF mutation affects a protein within the cell that reduces the cell's ability to function properly. This results in a build up of mucous within the lungs, lung dysfunction and possible death.
X-linked Disorders
X-linked disorders are due to mutations of genes on the X chromosome and have different patterns of inheritance due to their transmission on a sex chromosome and whether the embryo is male or female. Examples of X-linked diseases are the Fragile X syndrome and Duchenne muscular dystrophy.
