XYY syndrome
What is XYY syndrome?
XYY syndrome is a genetic condition that is caused by an extra chromosome. To
understand what XYY syndrome is, 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 up
parts of the body such as our muscles, our heart and our brain. Cells carry all the
information needed for our bodies to work. The information is contained in our genes. We
each inherit two copies of most genes, one copy from our mother and one copy from our
father. Genes act like a set of instructions, controlling our growth and how our bodies work.
Genes are responsible for many of our characteristics, such as our eye colour, blood type
or height.
Genes are carried on thread-like structures called chromosomes. It is possible to see
chromosomes under a microscope. Picture 1 shows what they look like.
Picture 1. Chromosomes as they appear under a microscope
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Usually, each of us has 46 chromosomes in every cell. We inherit our chromosomes from
our parents, one set of 23 chromosomes from our mother and one set of 23 chromosomes
from our father. So we have two sets of 23 chromosomes, or 23 pairs. The pairs of
chromosomes are numbered according to size, starting with the largest chromosome,
chromosome 1, through to chromosome 22. The remaining pair are called the sex
chromosomes.
Picture 2. The chromosomes arranged according to size; chromosome 1
is the largest. The two chromosomes at lower right are the sex
chromosomes.
There are two kinds of sex chromosome, one called the X chromosome and one called
the Y chromosome, and they control whether a person is male or female. Females usually
have two X chromosomes (XX). A female inherits one X chromosome from her mother and
one X chromosome from her father. Males usually have an X chromosome and a Y
chromosome (XY). A male inherits an X chromosome from his mother and a Y
chromosome from his father.
XYY syndrome
XYY syndrome affects only boys and is caused by an extra Y chromosome. A boy or man
with XYY syndrome has one X chromosome and two Y chromosomes (47,XYY), rather
than just one X chromosome and one Y chromosome as normal (46,XY).
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It may happen that a boy or man with XYY syndrome is a mosaic, meaning that some of
his cells contain the normal number of sex chromosomes (XY) and some cells contain an
extra Y chromosome (XYY). The effects of XYY syndrome in a mosaic depend upon the
proportion of XY cells to XYY cells throughout his body.
What causes XYY syndrome?
The majority of parents with a son who has XYY syndrome have a normal number of
chromosomes themselves. The extra Y chromosome occurs either during the making of
the father’s sperm or shortly after an egg and sperm come together to make a baby, at
conception. It occurs by chance and is not caused by anything that the parents did or did
not do.
Will it happen again?
It is very unlikely that XYY syndrome will affect another child of the same couple. Some
parents decide to have a test in a future pregnancy to check the chromosomes of the
baby. Testing can be discussed at a genetics clinic.
There does not appear to be an increased risk of a man with XYY syndrome passing this
on to his sons.
How does an extra Y chromosome affect a person?
The effect of having an extra Y chromosome can be very varied, but the majority of people
with XYY syndrome lead normal lives. They go to ordinary schools, have successful
careers, marry, have children and live until old age. Most people with an extra Y
chromosome will never know they have this, as they never have a reason to get their
chromosomes checked. However, some people with XYY syndrome may experience
problems such as learning difficulties and/or behavioural problems.
 First year of life
Boys born with XYY syndrome look and behave just like other babies. They are not
unusual in weight or length. Problems at birth are no more common than in any other
children.
 Age 1 to 5 years
Boys with XYY syndrome usually sit, crawl and walk at the usual time, but they may be
slower to learn to talk than the average child. About half of boys with XYY syndrome
have some delay in speech. In these boys, speech therapy can be helpful, starting by
around 3 to 4 years of age. With help, the problem usually disappears after a few
years.
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 Boys at school (5 to 16 years of age)
Boys with XYY syndrome tend to be taller than other boys of their age. This can mean
that more is expected from them than from other children of the same age. They have
a wide range of abilities, just as all children do. The majority of boys with XYY
syndrome are within the normal range for intelligence. However, research has shown
that the average intelligence of boys with XYY syndrome is slightly lower than that of
boys with the normal number of chromosomes. It is possible to have above average
intelligence and have XYY syndrome. About half of all boys with XYY syndrome may
need some extra help at school, but the majority manage well at mainstream school.
When there are learning difficulties, these tend to involve language and adapting to the
school environment.
Some may have behaviour problems such as temper tantrums, which may require
specialist support. These problems usually respond well to treatment. Some boys are
described as easily distractible and more active physically. If parents are aware of this
and can channel the child’s energies constructively, this does not need to be a
problem. Aggression is not more frequent than in other children. The boys seem to
cope less well with conflict and stress than their unaffected brothers and sisters.
A strong, supportive home environment is especially important for affected boys.
There is no increased risk of any serious mental illness.
Growth in boys with XYY syndrome
Boys with XYY syndrome grow slightly faster than average in childhood, and their
average height is 188cm (6 feet 3 inches). Puberty is normal and at the expected time.
Hormones are normal and no special treatment is needed. Homosexuality is no more
common than in the general population.
Boys with XYY syndrome may go on to further education, but are less likely to do so
than their brothers and sisters.
Boys with XYY syndrome are not more likely to get a criminal conviction than other
boys of the same intelligence level. Boys with XYY syndrome are not more likely to go
to prison or young offenders’ institutions than other boys.
Adulthood
The majority of men with XYY syndrome, around 75%, are in employment, in a wide
range of jobs.
Just as many men with XYY syndrome marry, have normal sex lives and have
children, as men with the normal number of chromosomes. They also do not appear to
be at any higher risk of having a son with XYY syndrome themselves.
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Where can I get more information about XYY syndrome?
This is only a brief guide to XYY syndrome. More information can be obtained from your
local regional genetics centre or from these addresses:
Unique- The Rare Chromosome Disorder Support Group
PO Box 2189
Caterham
Surrey
CR3 5GN
Telephone: 01883 330766
Email: info@rarechromo.org
Web:www.rarechromo.org
The Genetic Interest Group
Unit 4D, Leroy House
436 Essex Rd
London
N1 3QP
Telephone: 020 7704 3141
Email: mail@gig.org.uk
Web: www.gig.org.uk
Contact a Family
209-211 City Rd
London EC1V 1JN
Telephone: 020 7608 8700 FAX: 020 7608 8701
Helpline 0808 808 3555 or Textphone 0808 808 3556 (Freephone for parents and families,
10am-4pm, Mon-Fri)
Email: info@cafamily.org.uk
Web: www.cafamily.org.uk
This edition prepared in June 2005
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Glossary (difficult words and their meanings): XYY syndrome
This glossary is intended only to explain terms used in the information: XYY syndrome
Words shown in bold are defined elsewhere in the glossary.
(46,XY). Scientific abbreviation for the chromosomes usually found in men and boys.
There are 46 chromosomes, including an X chromosome and a Y chromosome.
(47,XYY). Scientific abbreviation for the chromosomes usually found in men and boys
with XYY syndrome. There are 47 chromosomes, including an X chromosome and two
Y chromosomes.
cell . The human body is made up of millions of cells, which are like building blocks. There
are 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 (except
for eggs in women and sperm in men) contains all the body’s genes.
chromosomes. Thread-like structures which can be seen under the microscope and
contain the genes. Usually people have 46 chromosomes in every cell. There are two sex
chromosomes and 22 other pairs of chromosomes. Twenty-three chromosomes come
from the mother, and twenty-three come from the father. One chromosome of each pair
comes from each parent. (As an analogy: a chromosome is like a book; a gene is like a
story in the book).
conception. When an egg and a sperm join together to make the first cell of a new
baby.
egg. The mother’s contribution to the cell which will grow to make a new baby. The egg
contains 23 chromosomes; one from each pair in the mother. A sperm also contains 23
chromosomes one from each of the father’s pairs. The egg joins with a sperm to make a
complete cell. A baby develops from this first cell.
gene. Information needed for the body to work, stored in a chemical form on
chromosomes. Changes in genes alter the information and this can change how the body
works. Most genes are in pairs, one from the mother and one from the father. The two
genes of a pair are at matching places on a pair of chromosomes. Genes on the X
chromosome and the Y chromosome of males are not paired with each other; they do
not match. (As an analogy: a gene is like a story in a book, changes in genes are like
changes in a story).
genetic. Caused by genes, concerning genes.
homosexuality. Sexual preference of men for men.
hormones. Chemicals made by the body to control particular tasks such as the start
and progress of puberty and the making of sperm.
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mosaic. A person who has some cells with the usual number of Y chromosomes (XY)
for a man, and some cells with two Y chromosomes (XYY). The effect on the person
depends on the proportion of cells with one and two Y chromosomes.
puberty. Start of changes in the body, associated in boys with growth in height,
deepening of voice, growth of body hair and the start of production of sperm.
sex chromosomes. The X chromosome and the Y chromosome. The sex
chromosomes control whether a person is male or female. Females usually have two X
chromosomes. Males usually have one X chromosome and one Y chromosome.
speech therapy. Specialist help with learning to talk, for children who start to talk late,
or who make slow progress with speaking.
sperm. The father’s contribution to the cell which will grow to form a new baby. Each
sperm contains 23 chromosomes; one from each pair in the father. An egg contains one
chromosome from each of the mother’s 23 pairs. The sperm joins with an egg to make a
complete cell. A baby develops from this first cell.
syndrome. A group of features which occur together in affected people. Affected people
share some or all of the features of the syndrome.
temper tantrums. Outbursts of excessive temper.
X chromosome. One of the sex chromosomes. Females have two X chromosomes.
Males usually have one X and one Y chromosome. Males with XYY syndrome have an
extra Y chromosome.
XX. This represents the sex chromosomes of a female. Females have two X
chromosomes. One X chromosome is inherited from each parent.
XY. This represents the usual sex chromosomes of a male. Males have one X
chromosome and a Y chromosome. A male inherits his X chromosome from his
mother and his Y chromosome from his father. Males with XYY syndrome have an extra
Y chromosome.
XYY. This represents the sex chromosomes of a male with XYY syndrome. Males with
XYY syndrome have one X chromosome and two Y chromosomes. They have an
extra Y chromosome.
Y chromosome. One of the sex chromosomes. Males usually have one Y
chromosome and one X chromosome. Females have two X chromosomes. Males with
XYY syndrome have one X chromosome and two Y chromosomes.
This glossary is intended only for use by patients and families, with the genetic
information to which it refers.
This edition prepared in July 2005
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