With recessive genes (in which both mother and father have to contribute a 'positive' set of the same genes) are designed to work in precisely the same way that hemophilia does: One out of four offspring (on average) will have the condition, two will be carriers, and one will be neither sick nor a carrier. Klinefelter's syndrome, on the other hand, arises from a mechanical mistake that occurs on the genetic level during the process of the way in which egg and sperm come together to create a genetically whole zygote/fetus/baby. Under normal genetic conditions, an egg brings half of the needed genetic material to an individual. This female contribution is referred to as the X chromosome. Sperm, which also contribute half of an individual's genetic material, can be either X or Y in terms of chromosomal type. An X sperm combines with the X chromosome of an egg to produce a girl; a Y sperm chromosome combines with the X chromosome of an egg to produce a boy (Klinefelter syndrome, 2007).

This process works very smoothly most of the time. However, sometime, when sperm are being produced, something breaks down in the process and instead of a sperm being formed as either an X chromosome sperm or a Y chromosome sperm, the sperm's own chromosome does not divide properly and the sperm has both an X and a Y chromosome. When this happens, a boy is born who has Klinefelter's syndrome, which is the genetic expression or consequence of a boy who has not the usual XY genetic structure of males but a structure of XXY, which can in very general terms be seen as something of a genetic combination of male and female genetic structures (Samango-Sprouse, 2010).

This mixture at the genetic level of male and female structures is reflected in the physical condition of males with Klinefelter's syndrome. While they are externally male (that is, they have a penis and testicles)...

Thus males with Klinefelter's syndrome are likely to have low rates of fertility because the normal functioning of male fertility mechanisms are interfered with by lower rates of male sexual hormones and higher-than-usual levels of female sexual hormones.
Likewise, males with Klinefelter's syndrome may have breast development that in key ways resembles female breasts rather than 'normal' male pectoral development. Men with Klinefelter's syndrome are also likely to have relatively little body hair and to have a "boyish" or "youthful" appearance (Simm & Zacharin, 2006). However, it is also the case that many individuals do not show any evidence (or very little evidence) of their underlying genetic condition. Thus, while this genetic condition occurs in as many as one out of every 500 males, the condition remains relatively unknown (Simm & Zacharin, 2006). Most men with the condition simply live with it, some seeking treatment in the form of testosterone that may increase fertility. But mostly, one suspects, they adjust to a condition that is endurable and that -- because of the genetically mechanical way in which it occurs -- they know that they cannot pass on to their children.