That is, adolescents with Klinefelter syndrome develop breasts, have higher voices than would be expected, and have less facial and body hair than most men. In addi- tion, although the penis of a male with XXY chromosomes is anatomically normal, the adult size is much smaller than for a male born with XY chromosomes. Testos- terone replacement therapy can encourage the development of more typically male physical characteristics. However, this therapy has serious risks, such as the possibility of liver damage.
By contrast, some zygotes have inherited only one sex chromosome, an X. In such cases, where there is just one (female) sex chromosome, the zygote is described as having XO sex chromosomes. The O refers to the missing chromosome. This very rare chromosomal abnormality results in Turner syndrome, which occurs in about 1 in 2,000–5,000 births (Intersex Society of North America, 2008). These children are born with normal vaginas. However, their ovaries do not develop normally, so they have reduced amounts of estrogen. As a result, during puberty, they do not show many of the typically female secondary sex characteristics, such as breast growth. Estrogen replacement therapy during adolescence may improve breast development and even start menarche.
CONFLICtS IN HORMONES Another group of people experience intersexual- ity due to a conflict between their sex chromosomes and the presence of certain hormones. Because of this conflict, they exhibit some characteristics of another sex. For example, a zygote with XX sex chromosomes—genetically female—may be exposed to greater amounts of androgens in utero. People with this condition, called congenital adrenal hyperplasia (CAH), may be born with genitals that are not clearly male or female, or their genitals may look more like a penis (Dessens, Slijper, & Drop, 2005).
By contrast, a zygote with XY sex chromosomes—genetically male—may be born with what looks like a vagina. This type of intersexuality is known as andro- gen insensitivity syndrome (AIS), which happens in about 1 in 13,000 births. The XY chromosomes should cause development of the testes, but in AIS the cells of the body do not respond to the testosterone released by the testes. Because of this lack of response, the testes remain in the body, and the penis does not fully develop.
Hormonal abnormalities are the most common reason that newborns (about 1 in every 1,500) have genitals that are not clearly a penis or a vagina (American Psycho- logical Association, 2006). In these cases, should the child be assigned the sex that most closely aligns with the genitals or with the sex chromosomes?
One study investigated this question with 16 genetically male infants (with XY sex chromosomes) who were born with unclear genitals. The parents of 14 of the children had their children’s genitals surgi- cally altered to be female and raised the chil- dren as female. By early childhood, 8 of these 14 children had declared themselves to be males, as did the 2 children who did not undergo surgical intervention (Reiner & Gearhart, 2004). The results of this study suggest that biological sex is determined more by the sex chromosomes than by the appearance of the genitals and the assigned sex. For this reason, the Intersex Society of North America strongly urges people to avoid genital surgeries for children.
Given what you just learned about people who experience intersexuality, it should be clear that biological sex is not just male or female. Instead, biological sex can be viewed as a continuum that includes greater or lesser physical aspects of each sex (Figure 10.6). However, the story of Coy Mathis, the transgender child discussed