Sex-Chromosome Disorders

Errors occasionally occur at the first level of biological sex determination, and indi­viduals are born with one or more extra sex chromosomes or missing one sex chromo­some. More than 70 atypical conditions of the sex chromosomes have been identified. These irregularities are associated with various physical, health, and behavioral effects. We consider two of the most widely researched of these conditions: Turner’s syndrome and Klinefelter’s syndrome.

Turner’s Syndrome Turner’s syndrome is a relatively rare condition characterized by the presence of only one sex chromosome, an X (Knickmeyer et al., 2011; Rivkees et al., 2011). This condition is estimated to occur in about 1 in every 2,500-3,000 live female births (Morgan, 2007). The number of chromosomes in the fertilized egg is 45 rather than the typical 46; the sex-chromosome combination is designated XO. People with this combination develop normal external female genitals and consequently are classi­fied as females. However, their internal reproductive structures do not develop fully; ovaries are absent or represented only by fibrous streaks of tissue. Females with Turner’s syndrome do not develop breasts at puberty (unless given hormone treatment), do not menstruate, and are sterile. As adults, women with this condition tend to be unusually short (Ross et al., 2011; Zeger et al., 2011).

Because the gonads are absent or poorly developed, and because the hormones are consequently deficient, Turner’s syndrome permits gender identity to be formed in the absence of gonadal and hormonal influences (the second and third levels of bio­logical sex determination). Individuals with Turner’s syndrome identify themselves as female, and as a group they are not distinguishable from biologically normal females in their interests and behavior (Kagan-Krieger, 1998). This characteristic strongly suggests that a feminine gender identity can be established in the absence of ovaries and their products.

Klinefelter’s syndrome

a condition characterized by the presence of two X chromosomes and one Y chromosome (XXY) in which affected individuals have undersized external male genitals.

Klinefelter’s Syndrome A more common sex-chromosome error in humans is Kline­felter’s syndrome. This condition, estimated to occur once in about every 1,000 live male births (Intersex Society of North America, 2006), results when an atypical ovum containing 22 autosomes and 2 X chromosomes is fertilized by a Y-chromo — some-bearing sperm, creating an XXY individual. Despite the presence of both the XY combination characteristic of normal males and the XX pattern of normal females, individuals with Klinefelter’s syndrome are anatomically male. This condi­tion supports the view that the presence of a Y chromosome triggers the forma­tion of male structures. However, the presence of an extra female sex chromosome impedes the continued development of these structures, and males with Klinefel­ter’s syndrome typically are sterile and have undersized penises and testes. Further­more, these individuals often have little or no interest in sexual activity (Money, 1968; Rabock et al., 1979). Presumably, this low sex drive is related, at least in part, to deficient production of hormones from the testes.

120 CHApTER 5

At a Glance

■ TABLE 5.3	Examples of Atypical Prenatal Sex Differentiation
Syndrome	Chromosomal Sex	Gonadal Sex	Reproductive Internal Structures	External Genitals	Fertility	Secondary Sex Characteristics	Gender Identity
Turner’s Syndrome	45, XO	Fibrous streaks of ovarian tissue	Uterus and fallopian tubes	Normal female	Sterile	Undeveloped; no breasts	Female
Klinefelter’s syndrome	47, XXY	Small testes	Normal male	Undersized penis and testes	Sterile	Some femi­nization of secondary sex characteristics; may have breast development and rounded body contours.	Usually male, although higher than usual inci­dence of gen­der identity confusion
Androgen insensitivity syndrome	46, XY	Undescended testes	Lacks a normal set of either male or female internal structures	Normal female genitals and a shallow vagina	Sterile	at puberty, breast devel­opment and other signs of normal female sexual matura­tion appear, but menstruation does not occur.	Female
Fetally androgenized females	46, XX	Ovaries	Normal female	Ambiguous (typi­cally more male than female)	Fertile	Normal female (individuals with adrenal malfunction must be treated with corti­sone to avoid masculinization).	Usually female, but significant level of dis­satisfaction with female gender iden­tity; oriented toward tradi­tional male activities.
DHTdeficient males	46, XY	Undescended testes at birth; testes descend at puberty	Vas defer­ens, seminal vesicles, and ejaculatory ducts but no prostate; par­tially formed vagina	ambiguous at birth (more female than male); at puberty, genitals are masculinized.	Sterile	Female before puberty; become mas­culinized at puberty.	Female prior to puberty; majority assume tra­ditional male identity at puberty.

Males with Klinefelter’s syndrome tend to be tall and somewhat feminized in their physical characteristics; they might exhibit breast development and rounded body con­tours (Looy & Bouma, 2005). Testosterone treatments during adolescence and adult­hood can enhance the development of male secondary sexual characteristics and can increase sexual interest (Rogol et al., 2010; Wikstrom et al., 2011). These individu­als usually identify themselves as male; however, they often manifest some degree of gender-identity confusion (Mandoki et al., 1991).

Gender Issues