6.18: Life is possible with too many or too few sex chromosomes.

It is usually fatal to have one too many or one too few of the non-sex chromosomes. When it comes to the sex chromosomes, however, the situation is less extreme. Many individuals are born lacking one of the sex chromosomes or with an additional X or Y chromosome, and they usually survive. We can get a glimpse into the role of the sex chromosomes by looking at the physical and mental consequences of having an abnormal number of these chromosomes. In each of the cases below, the condition is caused by nondisjunction of the sex chromosomes during the production of sperm or eggs (FIGURE 6-39).

Turner Syndrome: X Approximately 1 in 5,000 females carry only one X chromosome (and no Y chromosome), exhibiting a condition called Turner syndrome, denoted as X_ (or sometimes XO). This is the only condition in humans in which a person can survive without one of a pair of chromosomes. Instead of having 46 chromosomes in every cell, these individuals have only 45. As common as Turner syndrome is, in 98% of the fertilized eggs in which this condition occurs, the egg is spontaneously aborted long before a fetus can come to term.

There are both physical and mental consequences of the absence of a second sex chromosome.

• Women with Turner syndrome are usually relatively short, averaging 4 feet 8 inches in height.
• They develop a web of skin between the neck and shoulders.
• The ovaries never fully mature, so the women are almost always sterile.
• The breasts and other secondary sex characteristics develop incompletely.
• Intelligence is usually normal, but some learning difficulties are common.

Klinefelter Syndrome: XXY An individual who has two X chromosomes is female; an individual with an X and a Y is male. But what happens when both of those conditions exist? An individual who carries two X chromosomes and a Y chromosome, a condition known as Klinefelter syndrome, develops as a male. This is because, as we saw earlier, the Y chromosome carries genetic instructions that cause fetal gonads to develop as testes; if these instructions are absent, the fetal gonads develop as ovaries. The extra X chromosome, however, does cause Klinefelter males to be somewhat feminized, although this effect can be reduced through treatments such as testosterone supplementation. Approximately 1 in 1,000 males have the genotype XXY, making this one of the most common genetic abnormalities in humans.

Klinefelter syndrome has some physical and mental consequences.

• Men with this syndrome have testes, but they are smaller than average. Because the testes are small, levels of testosterone are low and the men are almost always infertile.
• They develop some female features, including reduced facial and chest hair and some breast development.
• They have long limbs and are slightly taller than average (about 6 feet on average).
• They learn to speak at a later age than average and tend to have language impairments.
• Some individuals have further additional X chromosomes, with the karyotypes XXXY or even XXXXY. These males also exhibit Klinefelter syndrome but more frequently have mental retardation.

A hermaphrodite is an individual with functioning male and female reproductive organs capable of producing both male and female gametes. Often it is mistakenly assumed that a person with Klinefelter syndrome must be a hermaphrodite because he has both two X chromosomes (which would usually make an individual a female) and an X and a Y chromosome (usually making the individual a male). Hermaphroditism is common among invertebrates and occurs in some fish and other vertebrates, but contrary to urban legends, human hermaphrodites do not exist. Some men with Klinefelter syndrome may have some features of the opposite sex; however, they do not produce female gametes and so are not hermaphrodites. Individuals with sex characteristics that preclude definitive identification as male or female are called “intersex.”

XYY Males There is no official name for the condition in which an individual has one X chromosome and two Y chromosomes, although such individuals are sometimes referred to as “super males.” This chromosomal abnormality occurs in approximately 1 in 1,000 males (and in about 1 in 325 males who are 6 feet or taller). There are no distinguishing features at birth to indicate that an individual carries an extra Y chromosome, and the vast majority live their lives normally without knowing they have an extra chromosome in every cell.

Several consequences of the XYY condition have been well documented.

• XYY males are relatively tall (about 6 feet 2 inches on average).
• They tend to have moderate to severe acne.
• Their intelligence usually falls within the same range as that of XY males, although the average may be slightly lower.

Because, in the United States, about three to five times as many XYY males are found in prisons (as a percentage of the prison population) as in the population as a whole, there has been tremendous controversy over whether the extra Y chromosome predisposes individuals to criminal behavior. These observations were not part of randomized, controlled, double-blind experiments, however, so they are open to numerous alternative interpretations.

In one study, for example, researchers identified all male Danish citizens born in Copenhagen between 1944 and 1947 and obtained sex chromosome determinations for the 4,139 of these men who were taller than 6 feet 0.4 inches (184 cm). From this sample, they found that the level of intellectual functioning (as indicated by scores on an army selection test taken by the men) was significantly lower among the XYY males than among XY males. Based on this finding, they suggested that the increased representation of XYY males in prison was more likely to be due to the fact that males of lower intelligence are more likely to be apprehended, regardless of which chromosomes they carry. The controversy remains unresolved, and the vast majority of XYY males are not in prison.

Although an error during meiosis in either the father or the mother can cause Klinefelter or Turner syndrome, it is only an error in meiosis in a male that can give rise to an XYY child. Why? Because only males carry Y chromosomes. Consequently, an XYY child must receive both of his Y chromosomes from his father. This means that the father’s sperm cell must have contained two copies of the Y, a result of nondisjunction during meiosis in the father.

XXX Females Sometimes called “metafemales,” individuals with three X chromosomes occur at a frequency of about 1 in 1,000 women. Very few studies of this condition have been completed, although initial observations suggest that some XXX females are sterile but otherwise have no obvious physical or mental problems.