In most cases, when children are conceived, they inherit a chromosome from each parent that helps to determine gender. X chromosomes are female and Y are male. The female child has two X chromosomes (XX), and the male child receives an X and Y chromosome (XY). In some circumstances, occurring in between 1 and 500 to 1000 births, a male child receives an extra X chromosome, creating an XXY chromosomal structure.

XXY chromosomes, or more accurately, the XXY chromosomal structure or pattern, when recognized, can create varied symptoms. These are not always expressed and the XXY condition is not always recognized. The doctor discovering the presence of the XXY chromosome arrangement was Dr. Henry Klinefelter.

The presence of the extra X may be called Klinefelter Syndrome, or boys with the XXY structure are referred to as XXY males instead of describing their differences as a syndrome. It is really important to remember that boys with XXY chromosomes may have all or none of the features associated with XXY males.

Some indications of the XXY chromosomal structure may be noticed in early childhood. These can include physical developmental delays, like walking or talking later than average. Speech difficulties may continue, and children with an XXY structure may require speech therapy. Learning disabilities in early school, especially challenges reading, can also affect some of these children. Kids with this additional X chromosome may also be more physically awkward and some are more shy or passive.

If XXY males are not diagnosed in childhood, they may receive diagnosis during puberty. Testicles are usually smaller and do not increase in size, as they do with most boys during adolescence. Many males with this extra chromosome are infertile, though not all of them are. Though these children may actually be taller than most, they may lack the normal signs of developing puberty, and can have rounder bodies, more breast tissue, and scant to no development of hair on the body.

XXY males often benefit from testosterone treatment during puberty to help produce adolescent development of adult male features. Lack of this treatment does not mean that sexual behavior in life is impaired. Many XXY kids have normal sexual lives in later life, though some men may note slightly less interest in sex. The XXY chromosome pattern does not predispose men to be “feminine” or to "develop" homosexuality. Studies on this suggest homosexuality is just as common in men who have the more common XY chromosome arrangement.

Diagnosis, especially for those who exhibit key symptoms, can be helpful. Kids who are not aware that there are physical reasons for slight differences between themselves and others may feel they are “weird” or “different” than other developing boys. Education, treatment with testosterone as needed, and support from parents can be a great help.

Supplementation with testosterone can also help lower risk of certain diseases later in life. For instance, men who do not get this help may have a higher rate of developing osteoporosis. Men with XXY chromosomes are also more likely to be at risk for breast cancer since they have slightly more breast tissue than the average male.