XYY syndrome is a chromosomal condition which occurs only in males and is found with a frequency of 1 in 1,000. A chromosome is a rod-like structure present in the nucleus of all body cells, with the exception of the red blood cells. Chromosomes store genetic information. Normally humans have twenty-three pairs of chromosomes, forty-six chromosomes in total. The twenty-thirdrd pair, otherwise referred to as the sex chromosomes, store genetic information which determine our sex. A female has a XX pair and a male has a XY pair of chromosomes.
A male affected by XYY syndrome has an additional Y chromosome as well as the usual XY pair of chromosomes resulting in the formation of XYY. Sometimes the additional Y chromosome is present in only some of the cells of the body, but not all. This is referred to 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.
What are the symptoms?
The effect of having an extra Y chromosome in some or all cells varies between individuals. Some males with XYY syndrome show very few symptoms. The majority are never diagnosed whilst others may be more severely affected. Additionally, individuals may be differently affected by the severity of their features. It is not possible, therefore, to offer a precise prediction of the symptoms before or even immediately after the birth of each XYY boy.
XYY boys grow taller than average, they have a ‘growth spurt’ during childhood which results in an average height of 6’2″. 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 lowered intelligence scores for the group compared with XY males. They may have delayed speech development and have difficulties in communication. Boys with an extra Y chromosome seem to be at higher risk of having problems at school. However, regular assessment of educational achievement allows early intervention and helps to prevent secondary behavioural problems. Some XYY boys have obtained degrees at University.
Behavioural problems involve difficult and defiant behaviour which usually starts in childhood. Temper tantrums are common. It has been suggested that XYY males are predisposed to commit criminal acts more frequently than expected. Long-term follow-up studies are currently underway to shed more light on this issue and, to date, show that the frequency of conviction does not differ from that found in XY boys of the same level of intelligence.
Sexual development is normal including development of sex organs and of secondary sex characteristics. Fertility is not affected.
Inheritance patterns and prenatal diagnosis
XYY occurs sporadically and is not usually passed on from a XYY father to his sons.
Chorionic villus sampling at ten to twelve weeks or amniocentesis at about sixteen weeks is available during pregnancy but is usually carried out only in mothers who are thirty-five or older; there is no increase in the frequency of having a boy with XYY over age 35, hence many will not be recognised.