XYY syndrome also known as Jacob’s Syndrome and YY Syndrome is a rare chromosomal disorder that affects 1:1000 males.

People normally have 46 chromosomes in each cell. Two of the 46 chromosomes, known as X and Y, are called sex chromosomes because they help determine whether a person will develop male or female sex characteristics. Females typically have two X chromosomes (46XX), and males have one X chromosome and one Y chromosome (46XY) in each cell.

XYY syndrome is caused by the presence of an extra copy of the Y chromosome in each of a male’s cells. As a result of the extra Y chromosome, each cell has a total of 47 chromosomes instead of the usual 46, resulting in 47XYY. Some males with XYY syndrome have an extra Y chromosome in only some of their cells. This phenomenon is called 46XY/XYY mosaicism.

Who is affected

XYY syndrome is a rare chromosomal disorder present at birth that affects only males. It is estimated to occur in approximately one in 1,000 live births. It is estimated that only 12 percent of all males with XYY are ever diagnosed, due in part to the subtle and varying physical and psychological symptoms of affected males, and in part to a lack of training in medical schools and other graduate programs about sex chromosome aneuploidies and their symptoms, diagnosis and treatment. If a boy with XYY is not diagnosed prenatally, there are few other opportunities for diagnostic inquiry of a possible genetic disorder.


Most cases of XYY syndrome are due to a cell division error in the sperm prior to conception. Rarely, the cell division error occurs after conception resulting in a mosaic of cells with 46 chromosomes and 47 chromosomes known as 46XY/XYY. The exact cause for why these errors in cell division occur is not understood. XYY syndrome is not inherited. Researchers don’t believe that there’s any genetic predisposition to it. That is, men with XYY syndrome are not more or less likely than other men to have children with XYY syndrome

Signs and symptoms

Characteristics of XYY syndrome are often subtle, consequently, males with this condition are often undiagnosed or misdiagnosed. XYY is a spectrum with characteristics and symptoms varying widely between individuals. The most common physical difference is increased height, which usually becomes apparent after the age of five or six, and results in an average height of about 6 feet, 3 inches by adulthood. Some individuals with XYY also develop severe cystic acne during adolescence. Most males with XYY syndrome have normal production of the male sex hormone testosterone and normal sexual development, and they are usually able to father children. Besides the potential for increased height, most affected individuals typically have a normal physical appearance (phenotype).

Boys with XYY syndrome typically have normal intelligence, although, on average, IQ is 10 to 15 points lower than siblings. Affected boys may exhibit delays in reaching developmental milestones, symptoms include delayed development of motor skills (such as sitting and walking), weak muscle tone (hypotonia), speech and language delays, hand tremors or other involuntary movements (motor tics), seizures, asthma, flat feet (pes planus), fifth fingers that curve inward (clinodactyly) and abnormal side-to-side curvature of the spine (scoliosis). These characteristics vary widely among affected boys and men, with males unlikely to display all these symptoms.

Learning challenges have been reported in up to 50 percent of cases, most commonly linked to speech delays, language problems, with reading and maths difficulties being common due to an increased incidence of dyslexia.

In some cases, affected individuals develop behavioural problems such as an explosive temper, hyperactivity, impulsivity, defiant actions, emotional difficulties, or, in some cases, antisocial behaviour. There is a higher rate of attention deficit and hyperactivity disorder (ADHD), depression, anxiety and an increased risk for having an autism spectrum disorder (although often mild), which is a group of developmental conditions that affect communication and social interaction.

It is unclear why an extra copy of the Y chromosome is associated with tall stature, learning problems, and other features in some boys and men.


In the past, there were many misconceptions about this disease. It was sometimes called the super-male disease because men with this syndrome were thought to be overly-aggressive and lacking in empathy. Recent studies have shown that this is not the case. Although individuals with XYY syndrome have an increased risk for learning disabilities and behavioural problems, they are not overly aggressive, nor are they at an increased risk of any serious mental illness. Because these boys are at a higher risk for having learning disabilities, they may benefit from speech therapy, tutoring, and general awareness of the specific issues they struggle with. Although the first years of school may be more challenging for boys with XYY syndrome, they generally go on to lead full, healthy, and normal lives.


Prenatal testing is significantly increasing due to its availability and continued advancements. Many expectant parents are completely unaware that these tests can also identify X & Y sex chromosome variations. These tests can be obtained via analysis of the mother’s blood known as the Harmony test or NIPT. The results can give a percentage of likelihood of an XYY diagnoses. Diagnoses isn’t confirmed until further testing is obtained. The options available are, the analysis of amniotic fluid via amniocentesis or analysis of tissue samples from a portion of the placenta via chorionic villus sampling (CVS). These tests are invasive and can have a small risk of causing a miscarriage. Expectant parents have the option to wait until the baby is born and confirm a diagnosis through the baby’s blood sample.

A diagnosis of XYY syndrome in children and adults is often not common and often overlooked, since there is often no medical reason to test for XYY syndrome. Testing is made based upon a thorough clinical evaluation, a detailed patient history, and specialised tests (i.e., chromosomal analysis) that detect the presence of an extra Y chromosome (47,XYY karyotype). The most common type of testing is a specialised blood test known as karyotyping. Children and adults displaying a combination of speech and language delays, motor delays, difficulty particularly with reading, displaying impulsive and challenging behaviour with attention difficulties and have a tall stature should have a blood karyotyping carried out to confirm or rule out an XYY diagnoses


There’s no cure for XYY syndrome, but there are treatment options that can help address specific symptoms.

Finding services and a good health professional and early intervention team quickly is important and can greatly increase a child with an XYY diagnoses ability to live a healthier and more productive life. Options vary greatly depending on how old a child or adult is at the time of his diagnosis, whether he has noticeable symptoms, and the severity of those symptoms.

For children initiating early intervention if delays in motor or language are identified is vital and can make a huge positive impact on a child’s life.

Speech therapy, physiotherapy, occupational therapy, or assistance for learning disabilities in the school setting may be of benefit. In most cases, affected individuals are very responsive to early intervention and treatment, and problems may resolve altogether within a few years. Treatment of acne may help an affected individual’s self-image. Attention deficit and hyperactivity disorder, difficulties with social interactions, or other behavioural problems can be treated with therapy or medication the same as in individuals who do not have XYY.

For adults who have had a late diagnosis or no early intervention engaging with health professionals and allied health specialists can positively impact their lives. Occupational therapists, behavioural therapists, councillors, psychologists and speech therapists can work with the individuals to implement tools and strategies to help with community and workplace engagement.

Please refer to the links provided to read and learn more specific information on prenatal diagnoses, children, adults, education and early intervention.

With proactive management and effective treatment the impacts XYY/Jacobs Syndrome has on a child and individuals life can be greatly decreased. Most individuals with XYY lead happy and fulfilling lives.

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