There appears to be a slight association between the incidence of Klinefelter syndrome and increased maternal age. This association is less than that found with autosomal trisomies, such as Down syndrome. Parents should be aware that there is nothing they did or did not do to cause Klinefelter syndrome in their child. The primary function of the Y chromosome, even in the presence of more than one X chromosome, is to cause the formation of the testes and subsequent masculine development. However, the presence of two X chromosomes in affected males causes abnormal development of the testes and decreased levels of the male hormone, testosterone.
Puberty begins at the usual time and results in a normal size penis, but the testes are small.
Body hair may be either male or female in distribution, but is usually scant. They often have a slightly feminine fat distribution pattern with rounded hips being the most notable feature. As this may cause embarrassment and is likely to exacerbate an already fragile self esteem, surgery to remove excess breast tissue is usually recommended. This involves a relatively simple procedure and causes little scarring. Although some mosaic individuals are able to procreate, the majority of affected males and all nonmosaic 47,XXY individuals are infertile.