Basic genetics Indications Types of tests
A blood or semen sample are required for genetic screening (see practical information).
The blood or sperm sample
is 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.
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 so-called sex chromosomes. The normal pairs are sorted according to size and numbered 1 to 22. The 23rd pair, the sex chromosomes, is XY in males and XX in females.
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:
- specifically for men, a low sperm count in the ejaculate can be a reason to perform a genetic test. In cases where there are less than 5 million sperm present in every ml of ejaculate, a genetic cause will be found in 1 in 20 of the cases;
- all sperm donors are also genetically screened. In any case your Karyotype is determined;
- genetic screening will be performed on both partners in the following situations:
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 your genetic identity card, so to speak.
- The test may be performed on men with a strongly reduced sperm count (less than 5 million sperm per ml of semen) to try to find the reason for this. One possible genetic cause could be the Klinefelter syndrome.
- It is also a standard test for sperm donors. The analysis of the blood sample not only determines the blood type and rhesus factor, but also maps the external characteristics: eye and hair colour, skin type, build, etc.
When using donor sperm, we always ensure the donor is as similar to the couple as possible regarding blood group and appearance.
As previously discussed, our reproductive cells behave differently to other cells in the body. During the complex reduction division process, specific ' divisional abnormalities' may occur.
This is why a specialised test was developed - FISH stands for fluorescence in-situ hybridisation - a process which enables a restricted chromosome-analysis of sperm: Three chromosomes are examined. 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 men who have a family member with Cystic Fibrosis and men with azoospermia, due to an absence of the vas deferens at birth. 80% of these men will also be carriers of the faulty mucoviscidosis gene.
The purpose of this test is also to determine whether a low sperm count (less than 5 million per ml of semen) is due to a genetic defect.
There are two types of receptors on the Y-chromosome: one makes a man a man, whilst the other on the q-arm of the chromosome, is concerned with the regulation of sperm production. 5% of men with low sperm production have one or more receptors missing from the q-arm of the Y-chromosome (hence the name of the test). A son born to one of these men would also inherit the same genetic problem.