Handbook of Genetic Counseling/Balanced Translocation
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- 1 Introduction and Contracting
- 2 Outline session
- 3 Medical/Developmental History
- 4 Family History
- 5 Explain Chromosomes and Genes (using sample karyotype)
- 6 What is a translocation?
- 7 Why do translocations happen?
- 8 How Translocations Occur
- 9 Show Karyotype and Explain Specific Break Points
- 10 Balanced Translocations
- 11 What Are the Effects
- 12 What will results from parental chromosomes tell us
- 13 What can happen in offspring of people with balanced translocation
- 14 What sort of problems do unbalanced translocations cause?
- 15 Tests in Pregnancy for Translocation Carriers
- 16 Psychosocial Considerations
- 17 References and Patient Resources
Introduction and Contracting
- Can you explain in your own words what you understand about why you were referred to genetics?
- What are your main concerns for ______ at this time?
- Do you have any specific questions you would like us to make sure we try to answer?
- Take medical and family history
- Review the chromosome results with you
- Explain more about why we want to do chromosome testing on you (parents)
- Dr Viskochil will examine him
- We will then make sure to discuss any further questions or concerns
- follow form that I started filling out and confirm info
- 3 generation pedigree
- include standard questions don't forget miscarriages as they occur more frequently in people with balanced translocation
Explain Chromosomes and Genes (using sample karyotype)
- When the lab looked at _____ 's cells under a microscope they found a change in the chromosomes
- The change they found was called a translocation
What is a translocation?
- Transfer of chromosomal material between chromosomes
Why do translocations happen?
- We do not really know why - (Sort of. It happens because it is possible. And its possible because of the inherent nature of the genetic material.)
- Estimated that about 1 person in 500 has a chromosome translocation
- We know that our chromosomes seem to break and rejoin quite often, and it is only sometimes that it leads to problems
- These changes are out of our control, and nearly always translocations happen in either the egg or sperm cell before they join together
- Therefore they are not likely to be caused by anything during the pregnancy
How Translocations Occur
- Sometimes the chromosomes break during cell division and are usually repaired
- Sometimes two fragments may break off from two different chromosomes, and "swap" places
- This is called a reciprocal translocation (this is the type of translocation he has)
Show Karyotype and Explain Specific Break Points
- Sometimes helps to see the actual karyotype
- 46 means that the usual number of chromosomes is present
- Y chromosome is what makes him male
- T stands for translocation, where a piece of chromosome 2 and 21 exchanged places
- The rest just tells us more exactly where the chromosomes broke
- So arrows are pointing to where the chromosomal material exchanged places
- In most cases, there is no loss or gain of chromosomal material during the exchange process
- When the lab looked they didn't see any missing or extra pieces of chromosomes and they therefore think that this might be a balanced translocation
What Are the Effects
- Often people will have a balanced translocation and not even know because they don't experience any problems (this occurs in majority of cases of new balanced rearrangements Warburton 1991)
- Sometimes, apparently balanced translocation carriers show some characteristics or symptoms (Warburton showed that frequency of abnormality was 6.1% but confidence interval includes 3% in apparently balanced translocations)
- One explanation for this finding is that the break may have occurred in the middle of a gene that is important in growth or development and the gene no longer provides correct instructions because it is disrupted
- We do not know if this translocation is the explanation for the characteristics and health problems in ______
What will results from parental chromosomes tell us
- In order to determine what the translocation might mean it is important to do chromosomes on all parents of children with a translocation
- If a parent has the same translocation and they do not have any of the same symptoms or characteristics as the child then we cannot conclude that the translocation is the cause
- If neither parent has the translocation than it may explain some or all of the characteristics and health problems in ______, but would be difficult to know for certain
- Parental chromosomes will also be important because it will help us determine what risks might be to future offspring
What can happen in offspring of people with balanced translocation
- If one of the parents carries a balanced translocation, it is possible for a child of theirs to acquire a bit of extra chromosome material or a bit missing
- Either of these would be known as an UNBALANCED translocation
- This extra and or missing pieces of chromosome can unfortunately cause serious mental, physical and medical challenges in the child who inherits them
- Sometimes they cause a miscarriage, but not always
- However, it is also possible for these same parents to have a child with normal chromosomes, or a child with the same balanced rearrangement of their chromosomes as the parent
- Having a balanced translocation of your chromosomes does not mean you cannot have normal healthy children
- Factors that influence risks (not applicable in this case)
- Ascertainment - A family ascertained as a result of a previously affected child is seen to be at increased risk of recurrence since the imbalance has been proved viable.
- Risk of having an affected child is lower when a family is ascertained following for example repeated spontaneous miscarriage since in these cases the imbalance is more likely to be inviable
- Each time they have a child, there are several possibilities.
- they may have a child with normal chromosomes
- they may have a child with the same balanced rearrangement of the chromosomes as the parent
- they may have a child with an unbalanced rearrangement of the chromosomes which may cause mental and physical challenges
- the pregnancy may end in a miscarriage
What sort of problems do unbalanced translocations cause?
- Can be difficult to predict because it depends on:
- Which parts of chromosomes are missing or duplicated (aneuploidy for 4p, distal 4q and 9p for example all have a high degree of viability therefore relatively high risk)
- how large of pieces they are (if large less likely to be viable due to more extra or missing information)
- There are not usually the same breaking points for each translocation because it is random so we don't have any other children to compare this exact translocation to
- Nearly always have learning problems though if translocation is unbalanced
Tests in Pregnancy for Translocation Carriers
- amniocentesis This is usually done from 14 weeks in the pregnancy and the result normally takes about 2 weeks.
- Chorionic villus sampling (CVS) usually done between 10-12 weeks in the pregnancy, and the results take about 2 weeks.
- Information can be difficult to understand because translocations are not an easy concept
- Don't expect them to remember everything, we will provide a summary in a letter we will write
- People who have a balanced translocation of their chromosomes do not have a disease that can be "caught" by other people
- Guilt about possibly passing this on (not under our control can't cause or prevent)
- What do other family members know and do you plan on telling them
- Either parent considering having more children because this may alter their plans or could be upsetting to think they may have a recurrence risk if they didn't think so before