Robertsonian Chromosome Translocations

What is a chromosome translocation?

To understand what a chromosome translocationis, it is helpful to know something about genes and chromosomes.

What are genes and chromosomes?

Our bodies are made up of millions of cells. These cells are grouped together to make upparts of the body such as our muscles, our heart and our brain. Cells carry all theinformation needed for our bodies to work. The information is contained in our genes. Weeach inherit two copies of most genes, one copy from our mother and one copy from ourfather. Genes act like a set of instructions, controlling our growth and how our bodieswork. Genes are responsible for many of our characteristics, such as our eye colour, bloodtype or height.

Genes are carried on thread-like structures called chromosomes. It is possible to seechromosomes under a microscope. Picture 1 shows what they look like.

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Picture 1. Chromosomes as they appear under a microscope

2Usually, each of us has 46 chromosomes in every cell. We inherit our chromosomes fromour parents, one set of 23 chromosomes from our mother and one set of 23 chromosomesfrom our father. So we have two sets of 23 chromosomes, or 23 pairs. The pairs ofchromosomes are numbered according to size, starting with the largest chromosome,chromosome 1, through to chromosome 22. The remaining pair are called thesex chromosomes.

There are two kinds of sex chromosome, one called the X chromosome and one called theY chromosome, and they control whether a person is male or female. Females normallyhave two X chromosomes, (XX). A female inherits one X chromosome from her motherand one X chromosome from her father. Males have an X and a Y chromosome, (XY). Amale inherits an X chromosome from his mother and a Y chromosome from his father.

Picture 2. The chromosomes arranged according to size; chromosome1 is the largest. The last two chromosomes are the sex chromosomes.

3What is a Robertsonian chromosome translocation?

Robertsonian translocations occur when one chromosome becomes attached to another.Only five of the 23 pairs of chromosomes take part in Robertsonian translocations (theseare chromosomes 13, 14, 15, 21 and 22). Robertsonian translocations happen during themaking of eggs and sperm, or shortly after an egg and sperm come together to make ababy (at conception).

Picture 3.

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A Robertsonian translocation like the one shown in picture 3 is called a balancedtranslocation. This is because the chromosome material has been rearranged in such away that no important bits of chromosome material have been lost from the cell and noextra material has been acquired. There is still the same number of genes, but they havebeen moved to different places.

A person who carries a balanced Robertsonian translocation is not usually affected by it,and is often unaware of having it. The only time it may cause the person problems, iswhen he or she comes to have children. This is because the child may inherit anunbalanced form of the translocation.

Unbalanced Robertsonian translocations

If either parent carries a balanced translocation, it is possible for their child to inherit anunbalanced chromosome rearrangement. If this happens, the chromosome rearrangementin the child either has some extra chromosome material or has some chromosomematerial missing. This type of rearrangement is called an unbalanced Robertsoniantranslocation.

It is important that we have the correct amount of chromosome material and therefore thecorrect number of genes. If there is missing or extra chromosome material, the body haseither too little or too much genetic information. This can lead to a chromosome disorder.The seriousness of the disorder depends on exactly which chromosomes are involved. If ababy does have an unbalanced Robertsonian translocation, there is very often somedegree of learning disability and physical disability and this is often severe. When oneparent carries a balanced translocation and the other has normal chromosomes, there arefour possible combinations of chromosomes that a child of theirs could inherit. These areshown in Picture 4.

Two pairs ofnormalchromosomes

Robertsonian translocation:a chromosome from one pair has becomeattached to a chromosome from another pair

4Picture 4. Chromosome arrangements in children of a parent whocarries a balanced Robertsonian translocation.

Parents chromosomes Parents chromosomes

Chromosomes in parent carrying a Chromosomes in other parentbalanced Robertsonian translocation (normal arrangement)

Possible chromosome arrangements in their children

1. Normal 2. Balanced 3. Unbalanced 4. Unbalancedchromosomearrangementin a child

Robertsonian translocation in a child

A child who inherits either of the first two chromosome arrangements (1 and 2 in picture 4)is not likely to have problems related to its chromosomes. A child who inherits anunbalanced Robertsonian translocation may have significant learning disability andphysical disability.

If a parent has a balanced Robertsonian translocation will he or shealways pass it on?

Not necessarily, there are several possibilities for each child:

The child may inherit entirely normal chromosomes.

The child may inherit the same balanced rearrangement as the parent. In mostcases the child will not have any problems as a result of the translocation.

The child may inherit an unbalanced rearrangement of the chromosomes. This mayhave one of two outcomes. Either the pregnancy may end in miscarriage or thechild may be born with some degree of physical disability and learning disability.

5Sometimes a child can be born with a Robertsonian translocation although both parentschromosomes are normal. This is called a de novo (from Latin) or new rearrangement. Inthis case the parents are unlikely to have another child with a translocation.

Tests for Robertsonian chromosome translocations

Genetic testing is available to find out whether a person carries a Robertsoniantranslocation. This is done on a small blood sample. It is also possible to do a test duringpregnancy to find out whether a baby has a Robertsonian translocation. This is calledprenatal diagnosis.

What about other family members?

Once a carrier of a balanced Robertsonian translocation has been discovered in a family, itis usual for specialist staff at a genetics clinic to offer to see other family members. If theserelatives wish it, they can be tested to check whether they carry the translocation.Sometimes people find it difficult to tell other members of the family about thetranslocation. They may be worried about causing anxiety in the family. In some families,people have lost touch and it may feel difficult to contact them. Genetic counsellorsworking in regional genetics centres have a lot of experience with families in thesesituations and may be able to offer help in approaching other family members.

Points to remember

Balanced Robertsonian translocation carriers are usually healthy. Problems mayarise in the children of a balanced Robertsonian translocation carrier.

A translocation either is inherited from a parent or arises at the time of conception.

A translocation cannot be corrected it is present for life.

A translocation is not something that can be caught from other people. Thereforea translocation carrier can still be a blood donor, for example.

People often feel guilty about problems like this, which run in the family. It isimportant to remember that a Robertsonian translocation is not caused by anythingthat the person who has it did or did not do. It is not caused by anything that thepersons parents did or did not do.

A carrier of a balanced Robertsonian translocation can have healthy children

6Where can I get more information about chromosome translocations?

This is only a brief guide to Robertsonian chromosome translocations. More informationcan be obtained from your local regional genetics centre or from these addresses:-

Unique - The Rare Chromosome Disorder Support GroupPO Box 2189,Caterham,Surrey,CR3 5GNTelephone: 01883 330766Email: info@rarechromo.org Web:www.rarechromo.org

The Genetic Interest GroupUnit 4D,Leroy House,436 Essex Rd.,LondonN1 3QPTelephone: 020 7704 3141Email: mail@gig.org.uk Web: www.gig.org.uk

Contact a Family209-211 City Rd.,LondonEC1V 1JNTel: 020 7608 8700Fax: 020 7608 8701Helpline 0808 808 3555 or Textphone 0808 808 3556(Freephone for parents and families, 10am-4pm, Mon-Fri)e-mail: info@cafamily.org.uk Web: www.cafamily.org.uk

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7Glossary (difficult words and their meanings): RobertsonianChromosome Translocations

This glossary is intended only to explain terms used in the information: RobertsonianChromosome Translocations. Words shown in bold are defined elsewhere in theglossary.

balanced Robertsonian translocation. A Robertsonian translocation in whichthe small amount of chromosome material lost does not affect any important genes. Aperson with a balanced translocation is not affected by it.

carrier. A person who has a balanced Robertsonian translocation; all the genesneeded for the body to work are there, but the chromosome material is rearranged in atranslocation.

cell. The human body is made up of millions of cells, which are like building blocks. Thereare many specialised types of cells. These include skin cells, brain cells, and blood cells.Cells in different parts of the body look different and do different things. Every cell (exceptfor eggs in women and sperm in men) contains all the bodys genes.

chromosomes. Thread-like structures which can be seen under the microscope andcontain the genes. Usually people have 46 chromosomes in every cell. There are two sexchromosomes and 22 other pairs of chromosomes. Twenty-three chromosomes comefrom the mother, and twenty-three come from the father. One chromosome of each paircomes from each parent. (As an analogy: a chromosome is like a book; a gene is like astory in the book).

conception. When an egg and a sperm join together to make the first cell of a newbaby.

de novo. Phrase from the Latin language, meaning from new. Used to describe atranslocation which is new in a child, when both the childs parents have normalchromosomes.

egg. The mothers contribution to the cell which will grow to make a new baby. The eggcontains 23 chromosomes; one from each pair in the mother. A sperm also contains 23chromosomes one from each of the fathers pairs. The egg joins with a sperm to make acomplete cell. A baby develops from this first cell.

gene. Information needed for the body to work, stored in a chemical form onchromosomes. Changes in genes alter the information and this can change how thebody works. Most genes are in pairs: one from the mother, one from the father. The twogenes of a pair are at matching places on a pair of chromosomes. Genes on the sexchromosomes of males are not paired; they do not match. (As an analogy: achromosome is like a book, a gene is like a story in the book, changes in genes are likechanges in the story).

genetic. Caused by genes, concerning genes.

8genetic counsellor. A specialist who gives information and support to people whohave a history of a genetic condition in their family, or who are concerned about their riskfor inheriting or passing on a genetic condition.

miscarriage. Early end to a pregnancy, before the baby can survive outside the womb.

prenatal diagnosis. Test during a pregnancy for the presence or absence of a geneticdisorder in the baby.

Robertsonian translocation. Chromosome translocation involving chromosomes13,14,15, 21 or 22.

sex chromosomes. The X chromosome and the Y chromosome. The sexchromosomes control whether a person is male or female. Females have two Xchromosomes. Males have one X and one Y chromosome.

sperm. The fathers contribution to the cell which will grow to form a new baby. Eachsperm contains 23 chromosomes; one from each pair in the father. An egg contains onechromosome from each of the mothers 23 pairs. The sperm joins with an egg to make acomplete cell. A baby develops from this first cell.

translocation. Rearrangement of chromosome material. Arises when a piece of achromosome is broken off and then reattached in the wrong place.

unbalanced Robertsonian translocation. A translocation in which thechromosome rearrangement either has some extra chromosome material or has somemissing chromosome material. May arise in the child of a parent with a balancedRobertsonian translocation.

This glossary is intended only for use by patients and families with the genetic informationto which it refers.

This edition prepared July 2005 Ref Glossary 21