The XYY karyotype was first described in a 1965 study of 197 males in a Scottish maximum-security prison. The study's findings indicated that 4.5% of the violent offenders studied had the XYY karyotype. Subsequent surveys by other researchers indicated a higher frequency of XYY males in prison and mental institution populations than in the general population. These findings lead early investigators to conclude that some forms of violent behavior and mental illness were associated with the presence of the extra Y chromosome and thus the XYY criminal syndrome was born.
The chromosome complement of a genotypically normal human includes 22 pairs of autosomal (body cell) chromosomes plus one pair of sex chromosomes. During mitosis, each of these 46 chromosomes are replicated and randomly distributed into daughter cells such that each new daughter cell contains 46 chromosomes. The reductive form of cell division, meiosis, results in the production of daughter cells that contain half as many chromosomes as the original parent cell. At the completion of meiosis, each of four daughter cells contains 23 chromosomes representing 22 autosomal chromosomes and one sex chromosome. If a pair of chromosomes fails to separate during anaphase, a process known as nondisjunction, then the resulting daughter cells contain alterations in chromosome number such that one daughter cell contains one extra chromosome while another daughter cell is missing one chromosome. Alterations in chromosome number often produce genotypic and phenotypic changes in offspring. The overall incidence of sex chromosome anomalies--extra or missing sex chromosomes--is 1 in 400 live male births and 1 in 650 live female births. One of the most forgiving of all sex chromosome mishaps is also one of the most misunderstood, the XYY male.
Approximately 1 in 1,000 males has an extra Y chromosome. Advances in molecular techniques leading to a more comprehensive understanding of chromosomes and the genes they contain, as well as careful review of experimental design and methodology of earlier studies, have caused scientists to re-evaluate their positions on the perceived impact of the extra Y chromosome. More than twenty-five years after Jacobs study, researchers have determined that beyond the XY pairing region--a region at the tip of the short arm of the Y chromosome that is homologous to a region at the tip of the short arm of the X chromosome--and a testes determining factor (TDF), the Y chromosome contains very few genes. Therefore, an extra Y chromosome is expected to have milder effects on phenotype than extra autosomes.
Recent studies have shown that there is no significantly increased risk for aggressive or antisocial behaviors in XYY males. Most adult XYY males are fertile, of average intelligence, have a tendency to be tall, have severe acne as teenagers, and have minor birth defects such as crooked eye or slight outturning of the elbows. Studies also indicate that XYY boys are at increased risk for motor delay and learning disabilities in language and reading skills. These deficiencies are consistent with up to 15% of children in the general population.
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