Genetic Screening
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Gene card (caryotyping) of a man. |
Basis genetics
A blood sample is required in order to perform Genetic screening. For more information see
blood tests. In one case a sperm sample is also required (see
sperm sample).
The blood or sperm 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.
Basis genetics
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 affect only (or mainly) males is all to do with 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, but which 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, see
PGS/PGD.
Indications
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:
- Specifically in men, a low sperm count can be reason enough 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;
- if you wish to become a sperm donor genetic screening will be performed and your caryoptype determined;
- genetic investigations will be performed on both partners of every couple 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 upon the DNA from which the chromosome is constructed).
Cytogenetic testsTo perform this test, white blood cells (lymphocytes) 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 lymphocytes condense into visual structures - chromosomes. At this moment we can see how many chromosomes there are and whether they appear normal.
- Caryotype testing [blood test]
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.
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This caryoptype illustrates an exchange (trans-location) of genetic information between chromosomes 1 & 10. |
- The test may be performed on men with a 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 Klinefelter syndrome.
The test is routinely performed in men wishing to become a
sperm donor. This enables appearance characteristics to be determined , such as eye and hair colour, skin type, build etc...
When using donor sperm, it is always ensured that the donor is as similar to the couple as possible regarding blood group and appearance.
- FISH-analysis [sperm sample}
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.
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FISH-analysis of sperm cells. |
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.
DNA-screening [blood test]
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.
- CFTR-gene (mucoviscidose gene) [blood test]
One Belgian in 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 performed on men who have a history of Cystic Fibrosis in their family, or who have azospermia due to an absence of the vas deferens at birth. 80% of these men will also be carriers of the faulty mucoviscidose gene.
- Yq-microdeletion test [blood test]
The purpose of this test too is 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 receptor 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. In 5% of men with low sperm production, there are 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.