Chromosomes are minute thread-like structures found inside the cells of our bodies. Each chromosome consists of two parts: the short arm “p” and the long arm “q”. On the chromosomes there are between 50,000 and 100,000 genes. These genes determine the individual hereditary characteristics of each person i.e. height, eye colour, blood group. See Fig 1
A human cell contains 46 chromosomes and these are arranged into 23 pairs. A human egg or sperm contains only one chromosome from each pair. Therefore, there should be 23 of the mother’s chromosomes in each egg and 23 of the father’s chromosomes in each sperm, the resultant fertilised egg will contain 46 chromosomes (23 pairs); one chromosome of each pair came from the mother and the other chromosome from the father. This is the unique “blueprint” for the individual baby. See Fig 2
Figure 2. The fertilised egg divides repeatedly to become first a ball of cells and then an embryo.
A karyotype or picture of chromosomes is prepared from a sample of blood. The blood cultures are grown for three days and the thread-like chromosomes in the cell are released on to a slide and stained with special dyes before being photographed, cut out and arranged in order of size to make study possible. See Fig 3
Figure 3. This is a karyotype of a male with the correct number of chromosomes.
Trisomy – How it occurs
The word trisomy is from the Greek word tri (three) and means that there are three identical chromosomes present in each cell instead of the usual pair. In trisomy 13 (Patau’s syndrome) an extra chromosome number 13 is present in each cell. See Fig 4
Figure 4. The karyotype of a male with Trisomy 13.
In trisomy 18 (Edwards’ syndrome) an extra chromosome number 18 is present in each cell. See Fig 5
Figure 5. The karyotype of a female with Trisomy 18.
The extra chromosome may come from either the mother’s egg or from the father’s sperm during cell formation, or when the cells of the fertilised egg do not divide correctly during cell division. See Fig 6
Figure 6. Here you can see that the chromosomes did not divide correctly. The result is an extra chromosome or trisomy. Trisomy may occur in either the mother’s egg or the father’s sperm.
Although the genes in the three chromosomes are normal, too much or too little genetic material in a cell affects every stage of the development of the baby. Therefore the blueprint for development is altered from the moment of conception.
In addition to trisomy 13 and trisomy 18, there are a number of related disorders as follows:
A very small percentage of babies are born, not with a complete trisomy but with a partial trisomy. This means that an extra piece of a chromosome is present. Babies with a partial trisomy may show some or all of the characteristics of the particular trisomy. This depends on the precise amount and nature of the additional chromosomal material.
In a balanced translocation, the normal number (46) of chromosomes are present in each cell but some or all of the material from one chromosome may be located on to a different chromosome. There is no loss of chromosomal material. Babies with balance translocations are completely normal and their chromosomal rearrangement should have no implications regarding health or physical appearance. However, their children would be at risk of inheriting an unbalanced translocation.
This means that either
(a) an extra piece of chromosomal material is present in each cell, usually attached to another chromosome
(b) that a piece of chromosomal material is missing.
Balanced or unbalanced translocations can arise spontaneously where both parents have normal chromosomes.
However, if a parent has a balanced chromosome re-arrangement then there can be a higher chance of that person having a pregnancy which may be miscarried or also going on to have a pregnancy which is affected with an unbalanced chromosome translocation. The chance of these happening depends on the nature of the translocation and what has previously happened in the family.
In addition, where one person in the family carries a balanced translocation it is also possible that other members of the family, who are otherwise healthy, may also carry that translocation which may have implications in turn for them if they are thinking about children.
A deletion is a condition where some chromosomal material is missing from the normal chromosome. In the case of a condition denoted by 13q-, the long arm “q” of chromosome 13 is missing. In the case of 18p- the short arm “p” of chromosome 18 is missing.
This condition is a less common form of trisomy. It results from an incorrect separation of chromosomes after the first normal division of the fertilised egg. Therefore, some cells have an extra chromosome (47), while other cells have the normal number of chromosomes (46). The number of cells having the extra chromosome varies from baby to baby and therefore each baby with mosaicism is unique. These babies are often less severely affected compared to those with an extra chromosome in every cell.
Here the two ends of a chromosome have broken off. Rejoining of the broken ends results in the formation of a ring chromosome and the loss of two chromosome fragments from each end. Ring chromosomes are rare.
A genetic counselling service is available to all parents. Your GP, Obstetrician, or Paediatrician can refer you for counselling. Your GP may refer you for genetic counselling, if appropriate, to the National Centre for Medical Genetics, Our Lady’s Hospital for Sick Children, Crumlin, Dublin 12.
The genetic counsellor will:
- Discuss the results of your baby’s chromosome study
- Explain all aspects of the disorder
- Discuss the variable and less common forms of trisomy with the family
- Assess the risk of having a second baby with a chromosomal disorder
- Discuss the implications of the genetic diagnosis for any children or any other family members which the couple may have
- Answer any questions you may have.
In most cases, the genetic counsellor will be able to reassure you that Patau’s or Edwards’ syndrome is unlikely to reoccur.