Basic information on genetics
Cells, chromosomes and genes
The body consists of 300 trillion cells which all contain the same genetic information. This information is contained in the DNA (the chemical substance deoxyribonucleic acid) that is found in 23 pairs of chromosomes in the cell nucleus. One half of a chromosome pair is inherited from the mother and the other half is inherited from the father. At identical places (loci) these pairs of chromosomes carry information for one and the same characteristic. Each characteristic is therefore determined by the information of two genes of one pair of genes. Some tens of thousands of genes are distributed over the 23 pairs of chromosomes. These genes contain the coded data (the genetic code) for making proteins, the building-blocks of the human body.
Genetic code
DNA is partly made up of a chain of four chemical elements, namely adenine (A), cytosine (C), guanine (G) and thymine (T), always in different sequences. These four substances are regarded as being the 'letters' of the genome. These make 'words'. In each case a word consists of a specific combination of three of the four letters and forms the code for an amino acid. Amino acids are the building-blocks of the proteins. From this 'genetic code', the DNA contains the complete building plan for producing all the proteins of the body.
Test material and methods
In order to carry out genetic testing it is necessary to have cells of the person to be tested. Because all the cells of an individual contain exactly the same DNA, any cell can be used for this. Generally white blood cells are chosen because they can be obtained very easily from a blood sample. About 2 ml of blood are taken for this.
In cytogenetic diagnostics the number of chromosomes and their make-up are examined under the microscope. Abnormalities in the number or substantial variations in their structure (such as a chromosome from which a large piece is missing) can be determined in this way.
From a number of disorders it is already known that they are caused by a protein that is not functioning or is functioning poorly. From a so-called molecular diagnosis of these diseases the genetic code is read from the DNA for that protein, and is compared with the code for a normal protein in order to determine any differences.
Normal and abnormal genes
In the case of some genes only one normal version is known, which is denoted as the 'wild type'. Other genes display polymorphism. This means that different variants of the gene exist, which can all be considered as being normal. Variations in the gene for the protein keratin determine for example whether you have curly or straight hair.
Changes in the structure or the function of the genes can cause all kinds of abnormalities and illnesses.
Abnormal genes can be inherited from one of the parents or from both together and thus result in hereditary illnesses. Genes can also, however, become defective through faults that are not inherited, but which develop in life, often under the influence of external factors. Defective genes such as this can, for example, be the starting point for a cancerous process.
Monogenetic disorders
Defects in a single gene or pair of genes are clearly responsible for hundreds of already known - dominant, recessive or X-linked - inherited disorders, by far the most of which are fortunately rare.
Some of these so-called monogenetic disorders, such as cystic fibrosis or mucoviscidosis (1 in 3,000 births) and fragile X syndrome (a hereditary mental disorder; about 1 in 1,000 births) occur somewhat more frequently.
By detecting the gene defect, certainty can be obtained about the question of whether someone has or will have a hereditary disorder.
Multifactorial disorders
In the case of disorders that occur much more frequently, such as raised blood pressure (hypertension), cardiovascular diseases, cancer, diabetes, etc., usually it is not possible to establish a simple causal relationship between the disease and the presence of one defective gene.
These diseases occur as a consequence of a complex interaction of the effects of different genes (polygenetic heredity) with environment-related factors and influences. All this is denoted as being multifactorial heredity. The degree to which the genes of a certain individual also contribute to the occurrence of a disorder is known as 'genetic susceptibility'.
Proof for a genetically determined component of susceptibility to these disorders comes from a study of the family, mainly of identical and non-identical twins. For diabetes and raised blood pressure this genetic component of susceptibility has been very clearly demonstrated.
For most multifactorial disorders it is not (yet) known which genes are involved.