A blood sample is required for genetic screening.
A blood sample is taken in the Outpatient Clinic (see practical
) and sent for analysis to the Centre for Medical Genetics (CMG) in UZ Brussel. The CMG provides us with the results, which we then share with our patients at the consultation
Important to know
Our reproductive cells behave differently to other cells in the body:
- the latter are created by normal cell division or 'mitosis', whereas
- reproductive cells undergo a complex reduction process during which they retain half of the chromosomes from other body cells.
During this process certain divisional abnormalities can only be detected in the reproductive cells.
Important to know: our reproductive cells behave differently to other cells in the body. The latter are created by normal cell division or 'mitosis', whereas reproductive cells undergo a complex reduction process during which they retain half of the chromosomes from other body cells. During this process certain divisional abnormalities can only be detected in the reproductive cells.
The human body is made up of many different kinds of tissue, which together form every component of our being. These tissues are made up of individual cells, all of which have a specific function within that tissue. To be able to function properly each cell has a set of on board instructions. This 'recipe book' is in the form of 'chromosomes', receptors which contain genes. In other words, chromosomes are the carriers of our genetic information and are made of DNA.
Most healthy people have 46 chromosomes, or 23 pairs. There are 22 'normal' pairs and one pair of sex chromosomes. The normal pairs are sorted according to size and numbered 1 to 22. The 23rd pair are the sex chromosomes: XX in females and XY in males.
Genetic screening used to be standard in the blood analysis of patients. However, experience has taught us this is unnecessary in the majority of cases. That is why genetic tests are only performed if there is a history of hereditary defects in your family, or indeed in case of specific indications:
- reduced ovarian reserve (insufficient reserve of eggs);
- defective menstrual cycle;
- in case of egg donation, for a genetic check-up of the donor;
- after three unsuccessful IVF attempts, where fertilisation takes place, but the embryo fails to implant;
- unexplained infertility;
- repeated miscarriage (three consecutive pregnancies);
- prior to PGS/PGD (or genetic screening of embryo).
In the last four cases, both the man and the woman are genetically screened.
Types of tests
There are two specific types of genetic tests:
- cytogenetic; which examines the chromosome in its entirety, and
- molecular; concentrates on the DNA from which the chromosome is constructed.
To perform this test, white blood cells (lymphocites) are placed in a culture medium. At a pre-determined stage of mitosis - hence the name cytogenetic - the genetic carriers of the lymphocites condense into visual structures, the chromosomes. At this moment we can see how many chromosomes there are and whether they appear normal.
A cytogenetic test enables Karyotype screening to be performed.
Karyotype literally means the character of the chromosomes: amount, length and genetic content. The ingredients of our genetic identity card so to speak.
Sometimes this test is performed to look for a specific abnormality. For example, in a diagnosis of anovulation (absence of ovulation) where no hormonal explanation can be found, a cytogenetic test can reveal the presence of Turner Syndrome (X0 Karyotype). People who have this condition are missing an X-chromosome, which gradually leads to premature menopause.
As previously discussed, our reproductive cells behave differently to other cells in the body. They undergo a complex reduction division process, whereby they retain half of the chromosomes from other cells in the body. During this process certain divisional abnormalities can only be detected in the reproductive cells.
For this reason, a special test was developed. FISH stands for Fluorescent in situ Hybridisation, a process which enables a restricted chromosome analysis of eggs. Three chromosomes are examined by way of sample. However, the test is not often performed and only for certain indications, such as when an abnormal number of abnormal embryos develop as a result of IVF in a couple.
Molecular testing goes slightly deeper, i.e. into the DNA structure of the chromosomes to determine which chromosomes display a mutation or deletion of genes. The question here is: where is the receptor fault or where is information missing? This sort of investigation is only restricted to a small number of chromosomes and genes.
CFTR-gen (mucoviscidosis gene)
One Belgian out of twenty is a carrier of the mucoviscidosis or CFTR-gene (Cystic Fibrosis Transmembrane Regulator gene). If both partners are carriers, their baby will stand a 25% chance of developing Cystic Fibrosis, a serious lung disease where abnormally large amounts of mucous are produced.
This DNA test is therefore performed on women who have a family member with Cystic Fibrosis, or if there is any other suspicion they may be a carrier of the faulty gene.
This specialised DNA test is used in women to detect mutations affecting the X-chromosome which can cause mental retardation in boys. Particularly women who donate eggs are screened for this condition. This sort of mutation can gradually increase from generation to generation and from a pre-mutation phase, go on to develop into an actual mutation.