Results of Chromosome Tests
Genetics is the study of human cells, and a Clinical Geneticist will interpret the results of chromosome tests taken from blood, skin, bone marrow, or connective tissue. When tests during pregnancy have shown a chromosome abnormality the clinical geneticist, if consulted, will discuss the results with the family.
Depending on the particular abnormality affecting the unborn child, the parents may not always be given the definite answers they seek, and in some cases the information will be a mixture of the certain, the probable, the possible, and an element of ‘wait and see’. Just as people have different characteristics, they may be tall, short, have brown eyes, blue eyes etc. and suffer from a variety of different conditions; for example eczema or asthma, so babies will have individual differences although sharing the same syndrome.
‘No research has been undertaken to find out why different children live to different ages. They live to different ages because of different problems within their bodies.’
Ian Young Professor of Paediatric Genetics
Diagnosis of a related disorder
When a translocation is present it can be difficult to predict the effect onthe child as this depends on the exact amount of genetic material that is extra or missing. This is especially true when mosaicism is diagnosed.
Blood cells may show a particular percentage of trisomy cells, skin cells canshow a higher or lower percentage, and cells in key organs such as the brain cannot be examined.
A detailed ultrasound scan can show abnormalities of organs such as the heart or kidneys, but it is not always possible to predict how severe the effects will be.
Can partial trisomy be inherited?
A baby can have partial trisomy when the parents have normalchromosomes, but in rare cases a parent may be a carrier if he or she has a balanced rearrangement of their chromosomes. A balanced rearrangement is when a piece of one chromosome is attached to another without there being any extra or missing genetic material.
Above are two pairs of chromosomes showing a balanced rearrangement.The (carrier) parent is healthy and unaware of the different pattern in their chromosomes unless tests are performed. When the parent cell divides to produce an egg or sperm the following can happen:
• The baby inherits the normal chromosome from each pair of chromosomes and has normal chromosomes.
The baby inherits the chromosome with a piece missing and the chromosome with the extra piece attached, ie. both the rearranged chromosomes so overall there is no extra or missing genetic material. The baby is healthy but carries the balanced translocation which may affect future generations.
• The baby inherits the chromosome with an extra piece, ie. extra genetic material, and has a partial trisomy of that chromosome, (see partial trisomy).
• The baby inherits the chromosome with a piece missing, ie. there is genetic material missing and the baby has a deletion of that chromosome, (see deletions).
Other family members may carry the balanced translocation and a clinical geneticist can explain how a future baby might be affected. Larger hospitals usually have a Genetics Department, and to obtain a referral you can ask your Obstetrician or G.P.
‘Andrew has partial trisomy 18 as a consequence of his dad having a pericentric inversion on chromosome 18. We are given a 5-10% recurrence risk for any future pregnancy and we had amniocentesis during my second pregnancy which showed normal chromosomes. Amy was born by elective caesarean section when Andrew was 18 months old and is just fine.’
‘My two year old son has partial trisomy 18 and I have been told I carry the genetic trait that caused him to have this. At first I felt guilty but it helped to talk to others in the same situation and we have since had a healthy daughter.
'After the birth of our daughter who had a partial trisomy 13 we consulted a geneticist and the results of blood tests showed my husband has a balanced translocation. At first he felt it was his fault and it took us a longtime to talk about it. His father has been tested and is also a carrier.’