XYY syndrome

XYY syndrome is a chromosomal condition which occurs only in males. A chromosome is a rod-like structure present in the nucleus of all body cells, with the exception of the red blood cells. Normally humans have 23 pairs of chromosomes, 46 chromosomes in total. The twenty-third pair are referred to as the sex chromosomes: a female has a XX pair and a male has a XY pair.

A male affected by XYY syndrome has an additional Y chromosome. The additional Y chromosome may only be present in only some of the cells of the body – which is known as a mosaic form of XYY syndrome. The extent to which such an individual is affected by XYY syndrome depends upon the proportion of XYY cells to XY cells throughout the body.

In this article

What are the symptoms of XYY syndrome?

Some males with XYY syndrome show very few symptoms, whilst others may be more severely affected. Some men with XYY have mild symptoms and therefore are not diagnosed. XYY boys grow taller than average, they have a ‘growth spurt’ during childhood which results in an average height of 6 foot, 2 inches. In early childhood, XYY boys are very active, with good eating and sleeping patterns. During adolescence, they may experience severe acne.

In some cases, XYY males show learning difficulties, with slightly lower intelligence than XY males. They may have delayed speech development and have difficulties in communication.

It has been suggested that some XYY males display difficult and defiant behaviour, which usually starts in childhood. Temper tantrums are common and XYY males may be at higher risk of having problems at school. Sexual development is normal and fertility is not affected.

How is XYY syndrome diagnosed?

A diagnosis of XYY syndrome is made based upon a thorough physical examination, a detailed patient history, and tests to analyse the genetic make-up of the affected boy. Karyotyping involves staining chromosomes from a sample (eg a sample of cells from inside of the cheek) and viewing them under a microscope. In XYY males an extra chromosome will be seen in the cells.

How is XYY syndrome treated?

Management of the condition requires support for the emotional and behavioural issues that a male with XYY may experience. There is no cure for XYY syndrome. Boys diagnosed with the condition should have a speech and language assessment. Speech therapy, occupational therapy, or assistance for learning in the school setting may be of benefit. In most cases, affected males are very responsive to early intervention and treatment, and any issues may resolve altogether within a few years. Behavioural problems can be treated with therapy or medication the same as in individuals who do not have XYY syndrome.

Inheritance patterns and prenatal diagnosis

Inheritance patterns
XYY syndrome occurs sporadically (by chance) with no previous family history. It is not usually passed on from a XYY father to his sons.

Prenatal diagnosis
Chorionic villus sampling at ten to 12 weeks or amniocentesis at about 16 weeks is available during pregnancy, but is usually carried out only in mothers who are 35 years old or older.

Is there support?

Information and support in the UK for XYY syndrome is provided by Unique (see entry Chromosome Disorders).

If your child is affected by a medical condition or disability, we can help. Call our freephone helpline on 0808 808 3555 to get information, support and advice. We also offer emotional support for parents via our Listening Ear service.

We have a range of parent guides on aspects of caring for a disabled child in our resource library. You may also find our Early Years Support useful, which contains links to parent carer workshops and help for families going through the diagnosis process.

You can also meet other parents online in our closed Facebook group.


Last reviewed August 2014 by Professor G Butler, Professor and Consultant in Paediatric and Adolescent Endocrinology, University College Hospital and UCL Institute of Child Health, London, UK.

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