Other Forms Of PGD Testing
Testing for a translocation
In some people, part of one chromosome attaches to the end of another, or pieces of chromosomes may break off and be swapped around so although that person has a normal complement of chromosomes their children could inherit missing or extra pieces of chromosome. This is known as ‘unbalanced translocation’.
Embryos with unbalanced translocations usually do not implant, or will miscarry or, rarely, are born with severe abnormalities. We can use PGD to detect unbalanced translocations as well as errors of all other chromosomes in an embryo.
Some genetic conditions affect one gender, for example haemophilia and muscular dystrophy. Sometimes it is not possible to detect the exact genetic error that causes the disease, but PGD can be used to determine the gender of embryos, as well as errors of all other chromosomes, and only embryos of the required gender and with the correct number of chromosomes will be transferred. Gender selection is prohibited .
Single gene defects
A gene is a sequence of DNA (genetic material) on a chromosome with a particular function (humans have up to 20,000 different genes, arranged on 23 pairs of chromosomes). When the DNA sequence on a particular gene is altered, a genetic disorder, such as cystic fibrosis, may result.
To identify which embryos are unaffected by the single gene defect that is present in the family, a patient-specific test is developed by the PGD team. As these tests are couple-specific, they can take some time and effort to design and establish.
Some people have (or have had) a parent affected by a serious genetic condition, such as Huntington’s disease, that does not generally manifest until later in life. Many people do not want to have testing to determine if they have inherited that condition from their parent but at the same time do not want to risk passing it on to their own children.
In this situation PGD can be used to perform exclusion testing. In exclusion testing the genetic error itself is not tested for but embryos are tested to make sure they have not inherited the ‘at risk’ gene from the member of the family who has the condition.