XYY Syndrome

What is XYY Syndrome?

XYY syndrome (also known as Jacobs syndrome) is a genetic condition, when males have an extra Y chromosome. Humans have 46 chromosomes- 22 pairs of somatic chromosomes and 1 pair of sex chromosomes. The female karyotype is XX, but male- XY. In case of XYY syndrome, males are born with extra Y chromosome, making their karyotype XYY, and total count of chromosomes 47 [1].

XYY Syndrome

Chromosome count in case of XYY syndrome
Image source: ghr.nlm.nih.gov

XYY Syndrome – a boy or a girl?

Since XYY syndrome is an additional Y chromosome to male karyotype, genetically this person is considered to be male. Most men with this syndrome also develop normally[1].


Although there is an extra Y chromosome in every cell, this syndrome doesn´t cause any particular visible features. Men with XYY syndrome can be taller than average. Persons with this condition also have normal sexual development and they are able to father children.

This syndrome is associated with risk of cognitive disabilities. Patients have a higher risk of problems with learning, developing ability to speak and learn language. In some patient there is also delay in motor function development. Affected persons have low muscle tone, problems learning how to sit and walk, possibly tremors and motor tics. All of the mentioned characteristics are various amongst affected persons.

Some of males with XYY syndrome have been diagnosed with autism spectrum disorders, like Asperger´s syndrome, attention deficit disorder. Affected individuals sometimes present with behavioral issues, like uncontrollable temper and aggressively [1,2].

Causes and Genetics

XYY syndrome is caused by an additional male sex chromosome- Y chromosome. Not all affected patients have extra Y chromosome in all of their cells. This is called mosaicism- some cells have normal karyotype, while some have extra Y chromosome (XY/XYY).

XYY syndrome

This condition in most cases in not inherited. Extra Y chromosome occurs as a random event during spermatogenesis. When sperm cells are developing, the genetic material does not divide. This error is called nondisjunction. Error in sperm cell division results in sperm cell with extra Y chromosome. When fertilization happens, the female egg cell with X chromosome joins with male sperm cell with two Y chromosomes, resulting in karyotype XYY. As the cells split, all of the cells forming the embryo have extra Y chromosome.

Cell division process, during which sperm cells with extra Y chromosome occur
Image source:Wikipedia.org

XYY mosaicism occurs as a random event during early stages of embryonic development. It results in fetus having both cells with XY and XYY karyotype [1,2].


There are usually no symptoms suggesting this syndrome at birth. First signs can be when motor and language skill development is delayed. There are a number of other syndromes that can cause motor and cognitive development delays. In case there are no other signs or features, like, for example distinct facial features in Mowat-Wilson syndrome, or defects associated with CHARGE syndrome, XYY syndrome should be considered.

The exact diagnosis can be made by detecting an extra Y chromosome in chromosomal analysis and by evaluating the patient.

XYY syndrome can be detected by using diagnostic tests during pregnancy. Extra chromosomes can be detected by using a sample of amniotic fluid that is obtained via amniocentesis. Other option is to obtain a tissue sample from placenta – procedure called chorionic villus sampling.

Psychiatric evaluation, as well as langue assessment and reading assessment should be done if the patient presents with cognitive development delay [2,3].

Differential Diagnosis

Some syndromes present with similar symptoms to XYY syndrome. These syndromes are:

  • Klinefelter syndrome– patients have an extra X chromosome. This syndrome also presents with motor and cognitive development issues, some patients are taller than average.
  • Sotos syndrome is a genetic disorder, that is characterized by excessive growth. People with this syndrome are taller than average person. they also have a prominent forehead and long face. These patients also have mental development issues, as well as language and motor skill development issues.
  • Marfan syndrome is characterized by abnormally tall body, with long fingers. This genetic disease affects connective tissue [2].

Treatment and Management

There is no treatment for XYY syndrome. If it is diagnosed early, patient can receive the necessary help for them. In early childhood motor development issues can be dealt by using physical therapy. In later years, various therapies can be used: physical therapy, speech therapy and education therapy[3].


XYY syndrome occurs in around 1 in 1000 newborns.

People with this condition are able to live completely normal lives. This syndrome can also remain undiagnosed. If symptoms of motor and cognitive development delay are caught early, by using appropriate therapy, patients have a better chance to develop normally.
Men with XYY are able to father children. It is rare that these men have sons. The extra Y chromosome is usually eliminated during the process of spermatogenesis [1].

Can XYY syndrome be prevented?

Cell division with nondisjuction happens spontaneously and it is not inherited. There is no way to predict if the fetus will have XYY syndrome.

XYY syndrome and criminal behavior

In the 1970´s and 1980´s concerns grew about men with XYY syndrome to be associated with criminal behavior. Due to this hypothesis, a lot of parents chose to do an abortion if prenatal diagnostic tests found XYY karyotype. Early studies suggested that males with XYY syndrome are more likely to develop criminal behavior. Patients with this syndrome can have an aggressive behavior, but they are not more likely to become criminals. Studies in 1990´s concluded that there is not higher risk of these men being criminals than men in general population. The early studies are believed to have errors in sampling errors and inclination towards proving the hypothesis to be right [4].


  1. https://ghr.nlm.nih.gov/condition/47xyy-syndrome#resources
  2. https://rarediseases.org/rare-diseases/xyy-syndrome/
  3. http://www.healthline.com/health/xyy-syndrome#Treatment5
  4. http://www.scienceclarified.com/dispute/Vol-1/Are-XYY-males-more-prone-to-aggressive-behavior-than-XY-males.html

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