Chromosome Translocations Information Leaflet

Introduction

This leaflet is written for individuals who have a chromosome inversion and for members of their family. It is intended to explain what a chromosome inversion is, what it means for the individual, for their wider family. What are Genes and Chromosomes?

Genes are tiny 'packages' of vital information which influence our growth and development. Each person has approximately 20,000 pairs of genes, which work rather like special computer programmes, and determine features such as the colour of our eyes, how many fingers and toes we have, how tall and short we are, and so on. Genes are so small that they cannot be seen, even under a powerful microscope. They are arranged along little threadlike strands, rather like the way that beads are arranged along a string. These strands are called chromosomes, and are inside most of the cells in our body. Unlike genes, chromosomes can be seen under a microscope.

A person usually has 46 chromosomes in each cell, arranged in 23 pairs. We inherit one of each pair of chromosomes from our mother and the other from our father. This is how we inherit characteristics from our parents.

To help differentiate between chromosomes, each pair has been given a number. The number 1 chromosomes are the largest, the number two are the next largest and so on right down to the number 22 which are the smallest. The remaining pair (X and Y) are called the sex chromosomes and determine person is male (XY) or female (XX).

Image Source: https://cpmc.coriell.org

This is a picture of what chromosomes would look like if we arranged them into pairs in order of their size. This is called a ‘karyotype’

What are Chromosome Translocations?

Sometimes when cells are dividing during egg or sperm formation, or in the very early development of the baby, one or more chromosomes can break and if they do not rejoin in the correct position this causes an unusual arrangement of the chromosomes known as a translocation. There are two different types of translocation:

i) Reciprocal translocation

A Reciprocal translocation occurs when two fragments break off from two different chromosomes and swap places, as shown in this diagram.

ii) Robertsonian translocation

A Robertsonian translocation occurs when two whole chromosomes become stuck together, as shown in this diagram.

Image Source: www.cbs.dtu.dk

To understand why translocations can be important in health and development, it is helpful to understand the difference between BALANCED and UNBALANCED translocations.

Balanced Chromosome Translocation

If the chromosome material has been rearranged in such a way that no chromosome material has been lost or gained it is known as a balanced translocation . The health of a person who carries a balanced rearrangement of their chromosomes is NOT affected by it. However, it may become important when they come to have children as there is a possibility a baby can inherit an unbalanced translocation.

Unbalanced Chromosome Translocation

If one parent carries a balanced translocation, when they make their eggs or sperm it is possible for their child to inherit a rearrangement of their genetic material resulting in an extra piece of one chromosome and/or a missing piece of another chromosome. A child who has inherited either too much or too little genetic material will usually have physical and/or learning difficulties. If A Parent Has A Balanced Translocation Will He/ She Always Pass It On?

Not necessarily as there are several possible outcomes for the pregnancy:

1. The parent with the translocation passes their ‘normal’ chromosomes into the egg or sperm, so the baby will have the correct number of genes in the usual order.

2. A baby may inherit the same balanced translocation as the parent. In this case the child would be expected to be a healthy as they have all their genetic information present.

3. A baby may inherit an unbalanced form of the translocation. A child who has inherited too much or too little genetic material will usually have physical and/or learning difficulties.

4. The pregnancy may end in a miscarriage.

Therefore, it is possible for a person who carries a balanced translocation to have healthy children, however, their risk of having a child with a learning or physical disability is higher than average. The precise risk depends on where in the chromosomes the translocation has occurred.

If we know a parent carries a chromosome translocation there are tests available in pregnancy that enable us to check the chromosome pattern of the developing baby to determine if it is carrying the correct amount of genetic information. These tests are called amniocentesis and chronic villus sampling (CVS) and there are separate leaflets available which explains these tests in more detail.

Some couples may be eligible for preimplantation genetic diagnosis (PGD). PGD involves checking the chromosomes of embryos created through IVF. Only embryos with the correct amount of genetic material are transferred into the womb with the hope it develops into a baby. How Can Someone Find Out If They Carry A Translocation?

If you are aware someone in your family carriers a chromosome translocation you can request a referral into your local genetic service via your GP. Your genetic counsellor will arrange a blood test to look at your chromosome pattern.

If you need more advice about any aspect of chromosome inversions, you are welcome to contact: Cheshire and Merseyside Clinical Genetics Service Liverpool Women’s Hospital Crown Street L8 7SS Telephone 0151 802 5001 or 5002 Facsimile: 0151 702 4286

This leaflet can be made available in difference formats on request. If you would like to make any suggestions or comments about the content of this leaflet, then please contact the Patient Experience Team on 0151 702 4353 or by email at [email protected]

Liverpool Women’s NHS Foundation Trust Crown Street Liverpool L8 7SS Tel: 0151 708 9988 Ref: Cli_2018-25 Issue Date: 01/05/2017 Review Date: 01/05/2020 © Liverpool Women’s NHS Foundation Trust