What is XYY syndrome?

Quick Answer
Males with XYY syndrome bear an extra Y chromosome. The syndrome occurs at fertilization and represents one of several human sex chromosome abnormalities.
Expert Answers
enotes eNotes educator| Certified Educator
Risk Factors

Only males are at risk for XYY syndrome. Most cases of the disorder are not inherited but result from a random event during the formation of sperm cells.

Etiology and Genetics

All normal human cells contain forty-six chromosomes consisting of twenty-three pairs; one member of each pair is contributed by the female parent and one by the male. Of these forty-six chromosomes, two chromosomes, designated X and Y, are known as the sex chromosome pair. Individuals with an XX pair are female, while those with an XY pair are male. Unlike the other twenty-two chromosome pairs, the X and Y chromosomes are strikingly different from each other in both size and function. While the Y chromosome is primarily concerned with maleness, the X chromosome contains information important to both genders.

During formation of sperm and eggs in the testes and ovaries, respectively, a unique form of nuclear division, known as meiosis (or reductional division), occurs during cell division that halves the chromosome number from forty-six to twenty-three. Sperm and eggs thus carry only one member of each pair of chromosomes, and the original number will be restored during fertilization. Because females have an XX pair, their eggs can have only an X chromosome, while males, having an XY pair, produce sperm bearing an X or a Y chromosome.

A common genetic error during sperm or egg production is known as nondisjunction, which is the improper division of chromosomes between the daughter cells. Nondisjunction in the production of either gamete can result, at fertilization, in embryos without the normal forty-six chromosomes. XYY syndrome is one of several of these aneuploid conditions (possession of one or a few more or less than the normal number of chromosomes) that involve the sex chromosome pair. While Klinefelter syndrome (an XXY male) and Turner syndrome (an X female) are more widely studied and recognized genetic diseases, the XYY male occurs with a frequency of 1 in 1,000 male births in the United States, according to the National Institutes of Health. Caused by a YY-bearing sperm fertilizing a normal X-bearing egg, the XYY embryo develops along a seemingly normal route and, unlike most other sex chromosome diseases, is not apparent at birth. The only physical clue is unusually tall stature; otherwise, an affected male will be normal in appearance. The XYY male is also fertile, unlike those with aneuploidies involving other combinations of sex chromosomes, which usually result in sterility.


Boys with XYY syndrome are usually tall, have difficulties with language, and may have slightly lower intelligence quotients (IQs) than other members of their families. They may also develop learning disabilities, hyperactivity, attention deficit disorder, and minor behavioral disorders.

Interest in the association between aggression and the Y chromosome began in the years following World War II. Both psychologists and geneticists began intensive scrutiny of the genes that were located on the male sex chromosome. Men with multiple copies of the Y chromosome thus became the subjects of much of this research. Genetic links to violent, aggressive, and even criminal behavior were found, although many argued that below-average intelligence played a greater role. Many males with XYY syndrome do perform lower than average on standard intelligence tests and have a greater incidence of behavioral problems. The majority, however, lead normal lives and are indistinguishable from XY males.

The controversy surrounding this research began with a study at Harvard University that began in the early 1960s and ended in 1973 because of pressure from both public and scientific communities. The researchers screened all boys born at a Boston hospital, identifying those with sex chromosomal abnormalities. Because the parents of XYY boys were told of their children’s genetic makeup and the possibility of lower intelligence and bad behavior, critics claimed that the researchers had biased the parents against their sons, causing the parents to treat the children differently. The environment would thus play a greater role than genetics in their behavior. Subsequent research has shown that the original hypothesis is at least partially accurate. There is a disproportionately large number of XYY males in prison populations, and they are usually of subaverage intelligence compared to other prisoners. It must be emphasized, however, that the majority of XYY males show neither low intelligence nor criminal behavior.

Scientists, doctors, geneticists, and psychologists now agree that the extra Y chromosome does cause above-normal height, reading and math difficulties, and, in some cases, severe acne, but the explanation of the high prevalence of XYY men in prison populations has changed its focus from genes to environment. Large body size during childhood, adolescence, and early adulthood will no doubt cause people to treat these individuals differently, and they may in turn have learned to use their size defensively. Aggressive behavior, coupled with academic difficulties, may lead to further problems. Clearly, however, the majority of XYY males do well. The issue would be much easier to resolve if a YY or Y male existed, but because lack of an X chromosome results in spontaneous miscarriage, no YY or Y male embryo could ever survive.

Screening and Diagnosis

Identification of this disorder after a child is born requires that the child receive genetic testing or screening and is often discovered accidentally as a consequence of results from another genetic test. A pregnant woman’s cells can be examined by either amniocentesis or chorionic villus sampling to determine if her child will have XYY syndrome. In amniocentesis, a sample of cells is taken from the amniotic fluid; chorionic villus sampling examines cells in the placenta.

Treatment and Therapy

Men with XYY syndrome may require help in school to deal with learning disabilities and behavioral difficulties. Speech, physical, and occupational therapies can also prove beneficial. The support of family members is another important element in helping men cope with this disorder.

Prevention and Outcomes

There is no cure for XYY syndrome. However, help in school and a supportive family can reduce the educational and behavioral problems associated with the disorder. Most males with XYY syndrome lead normal lives.


Bryant, Daniel M., et al. "Sex Chromosomes and the Brain: A Study of Neuroanatomy in XYY Syndrome." Developmental Medicine & Child Neurology 54.12 (2012): 1149–56. Print.

Chen Harold. “XYY Syndrome.” Atlas of Genetic Diagnosis and Counseling. Totowa: Humana, 2006. Print.

Kim, Ina W., et al. "47 ,XYY an Male Infertility." Rev. in Urology 15.4 (2013): 188–96. Print.

Mader, Sylvia S. Human Reproductive Biology. 3d ed. Dubuque: McGraw-Hill Higher Education, 2005. Print.

Simpson, Joe Leigh, and Sherman Elias. “Sex Chromosomal Polysomies (47,XXY; 47,XYY; 47,XXX), Sex Reversed (46,XX) Males, and Disorders of the Male Reproductive Ducts.” Genetics in Obstetrics and Gynecology. 3d ed. Philadelphia: Saunders, 2003. Print.

Tamarin, Robert H. Principles of Genetics. Boston: McGraw-Hill, 2002. Print.

Templado, C., F. Vidal, and A Estop. "Aneuploidy in Human Spermatoza." Cytogenetic & Genome Research 133.2-4 (2011): 91–99. Print.

Vernice, Mirta, and Anna Cremante. “Life Span Development in XYY.” Life Span Development in Genetic Disorders: Behavioral and Neurobiological Aspects. New York: Nova Biomedical Books, 2008. Print.

Access hundreds of thousands of answers with a free trial.

Start Free Trial
Ask a Question