Genetic card (caryotype) of a woman.
A blood sample is required in order to perform Genetic screening.
The blood sample is sent 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 consultation.
Important to know: Our reproductive cells behave differently to other cells in the body. Other cells are created by normal cell division or 'mitosis', whereas reproductive cells undergo a complicated reduction process during which they retain half of the chromosomes from other body cells. During this process, certain divisional abnormalities can take place, which are only detectable in the reproductive cells.
The human body is made 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, 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 1-22. The 23rd pair, the sex chromosomes are known as XX in females and XY in males.
The fact that certain genetic sicknesses or abnormalities only (or mainly) affect males is due to the chromosomal structure. X-related abnormalities occur less in women than in men, because they have the tendency to avoid the second X chromosome. Therefore, it is possible for a female to be a carrier of a genetic abnormality, without being affected herself. This would be passed over to any male offspring she may have, as it is from the affected X chromosome that the development of the male embryo will be determined. A female baby will be unaffected, because she will acquire all the information she needs for her development from the normal second X chromosome.
For information concerning genetic screening of embryos, including sex determination thereof, see PGS/PGD
It used to be that genetic screening was performed on all of our patients. However, this proved to be unnecessary in the majority of cases. Therefore these examinations are now only performed if there is a history of genetic or hereditary defects or diseases, or indeed in the case of other specific indications:
This coloured chromosome chart illustrates an exchange (translocation) of genetic information between chromosomes one and seven.
- reduced ovarian reserve (insufficient reserves of egg cells);
- Defective menstrual cycle;
- after three unsuccessful IVF attempts, where the fertilization takes place, but the embryo fails to implant;
- by unexplained (idiopathic) infertility;
- by repeated miscarriage ( three in a row);
- prior to PGS/PGD, or genetic screening of embryo.
In the last four cases, screening is performed in both the man and the woman.
Types of tests
There are two specific types of genetic tests: cytogenetic (which examines the chromosome in its entirety) and molecular (concentrates upon the DNA from which the chromosome is constructed).
To perform this test, white blood cells (lymphocites) are placed in culture medium. At a pre-determined stage of mitosis (the process by which cells divide and the reason for the name cytogenetic), the genetic carriers of the lymphocites condense into visual structures - chromosomes. At this moment we can see how many chromosomes there are and whether they appear normal.
Determination of Caryotypes
A cytogenetic test enables caryotype screening to be performed. Caryotype 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 in order 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 caryotype). People who have this condition are missing an X-chromosome, which gradually leads to premature menopause.
As previously discussed, our reproductive cells behave in a different manner to other cells in the body. They undergo a complicated reduction division process, whereby they retain half of the chromosomes from other cells in the body. During this process, specific division 'faults' can occur, which therefore are only traceable in the reproductive cells.
For this reason, a special testing procedure was developed. FISH stands for Florescent in situ Hybridization, a process which enables a restricted chromosome-analysis of egg cells. Three chromosomes are examined. The test is not often performed and only for certain indications, such as when an abnormal number of abnormal embryo's 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 therefore: where are there receptor faults or omissions of information? This sort of investigation is restricted to a small number of chromosomes and genes.
In this machine certain genes are copied and visualized.
This machine determines the presence or absence of certain genes.
CFTR-gene (mucoviscidose gene)
One Belgian out of twenty is a carrier of the mucoviscidose 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 someone in their family who has Cystic Fibrosis, due to the possibility that they themselves could potentially be a carrier of the faulty gene.
This specialized DNA test is used in women to detect mutations affecting the X-chromosome which can cause mental retardation in boys. Women who donate egg cells are especially 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.