This paper examines Klinefelter's syndrome, a chromosomal condition affecting males who inherit an extra X chromosome, resulting in an XXY genetic structure. The paper begins by contrasting intentional genetic inheritance with unintentional chromosomal errors, then explains the basic mechanics of genetic inheritance using hemophilia as a reference point for recessive gene transmission. It then distinguishes the mechanism underlying Klinefelter's syndrome — a chromosomal non-disjunction error during sperm formation — from classical recessive inheritance. The paper concludes by describing the physical and hormonal effects of the XXY karyotype, including reduced fertility, breast development, and limited body hair, while noting that many affected individuals show few outward signs of the condition.
When individuals think about what they would like to pass on to their children, they often think of intangible traits such as personal courage or artistic talent. Such attributes no doubt have a genetic or inherited element — because all traits a human has can arguably be said to have some genetic component — but these are traits that are in large measure learned, even if many of them may be learned from family members to whom an individual is genetically related. Parents may also think about passing on traits that are clearly genetically based, such as a dimple in the chin or curly hair.
However, one of the things parents may not consider is their potential to harm their children in an entirely unintentional way. That is, parents may harm their children unwillingly by passing along a combination of chromosomes that together can cause lifelong problems. This paper examines one of the lesser-known genetic conditions that can occur when a child receives a particular genetic contribution from each parent, and how Klinefelter's syndrome results from a different general mechanism than that which underlies better-known conditions such as hemophilia (Klinefelter syndrome, 2007).
Human genetics is both immensely complex and, in its broad outline, very simple. The simple part is that each person inherits half of his genetic information from his mother and half from his father. (Masculine pronouns are used throughout this paper because, while genetic dynamics of course work in females as well as males, the condition examined here occurs only in males.) Understanding how the genetic material of parent generations affects that of their children is simple in its general outline, yet complex in the repercussions that the parental genetic contribution can have on a child.
Most well-known genetic diseases — although it is probably more accurate to call them "conditions" rather than "diseases," since they cannot be caught from another person or from the environment but must be inherited — result when parents each carry a single gene that, when combined with a copy of that gene from the other parent, causes the condition to move from potential to expressed. The condition of hemophilia illustrates how this works. If a mother carries only one of the genes an individual must have to develop hemophilia, she herself will not be a hemophiliac but will instead be a "carrier" — an individual who carries to future generations the genetic possibility of the condition (Klinefelter syndrome, 2007).
The same is true for the paternal set of genes. A father can be a carrier of genetic conditions, but so long as he has only half — not a full complement — of the genetic coding for hemophilia, he will not himself be a hemophiliac, though his children may be, if their mother is also a carrier. Two parents who are both carriers of the chromosome underlying a genetic condition like hemophilia will not necessarily have children with the condition. Rather, on average, one out of four of their children will have the condition, two will themselves be carriers, and one will be neither affected nor a carrier. This basic mathematics of genetic inheritance was one of the first important truths understood about genetics (Klinefelter syndrome, 2007).
It is important to note that while hemophilia can have devastating consequences for an individual, it is not a genetic mistake in a purely biological sense. Recessive genes — in which both mother and father must contribute a "positive" set of the same genes — are designed to work in precisely the way hemophilia does: one out of four offspring, on average, will have the condition, two will be carriers, and one will be neither affected nor a carrier.
The genetic mechanism that causes Klinefelter's syndrome is different, because it arises from a genetic mistake — an error in the way genetic inheritance works when it follows biologically "normal" rules. Klinefelter's syndrome arises from a mechanical mistake that occurs at the genetic level during the process by which egg and sperm come together to create a genetically complete zygote. 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 chromosomal type. An X sperm combines with the X chromosome of an egg to produce a girl; a Y sperm combines with the X chromosome of an egg to produce a boy (Klinefelter syndrome, 2007).
This process works smoothly most of the time. However, sometimes during sperm production something breaks down: instead of a sperm forming as either an X-chromosome sperm or a Y-chromosome sperm, the sperm's own chromosome does not divide properly, and the resulting sperm carries both an X and a Y chromosome. When this happens, a boy is born with Klinefelter's syndrome — the genetic expression of an XXY chromosomal structure rather than the typical XY structure of males. In very general terms, the XXY structure can be understood as something of a genetic combination of male and female chromosomal material (Samango-Sprouse, 2010).
"Physical traits resulting from extra X chromosome"
"How affected males manage and adapt to condition"
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