Our biological sex is determined at conception by the chromosomal makeup of the sperm (male reproductive cell) that fertilizes an ovum, or egg (female reproductive cell). Except for the reproductive cells, human body cells contain 46 chromosomes, arranged in 23 pairs (see 1 Figure 5.1). Twenty-two of these pairs are matched; that is, the two chromosomes of each pair look almost identical. These matched sets, called autosomes (AW-tuh-sohmes), are the same in males and females and do not significantly influence sex differentiation. One chromosome pair, however—the sex chromosomes—differs in females from that in males. Females have two similar chromosomes, labeled XX, whereas males have dissimilar chromosomes, labeled XY.
As noted, the reproductive cells are an exception to the 23-pair rule. As a result of a biological process known as meiosis, mature reproductive cells contain only half the usual complement of chromosomes—one member of each pair. (This process is necessary to avoid doubling the chromosome total when sex cells merge at conception.) A normal female ovum (or egg) contains 22 autosomes plus an X chromosome. A normal male sperm cell contains 22 autosomes plus either an X or a Y chromosome. If the ovum is fertilized by a sperm carrying a Y chromosome, the resulting XY combination will produce a male child. In contrast, if an X-chromosome-bearing sperm fertilizes the ovum, the result will be an XX combination and a female child. Two X chromosomes are necessary for internal and external female structures to develop completely. But if one Y chromosome is present, male sexual and reproductive organs will develop (Harley et al., 1992).
Researchers have located a single gene on the short arm of the human Y chromosome that seems to play a crucial role in initiating the sequence of events that leads to
testes
Male gonads inside the scrotum that produce sperm and sex hormones.
the development of the male gonads, or testes. This maleness-determining gene is called SRY (Marchina et al., 2009; Nishi et al., 2011).
Findings from a study conducted by scientists from Italy and the United States suggest that a gene or genes for femaleness also exist. These researchers studied four cases of chromosomal males with feminized external genitals. All these individuals were found to have XY chromosomes and a working SRY (maleness) gene. Three of the four individuals exhibited clearly identifiable female external genitals; the fourth had ambiguous genitals. If the maleness gene was the dominant determinant of biological sex, the external genitals of these individuals would have developed in a typical male pattern. What, then, triggered this variation from the expected developmental sequence? Examination of these individuals’ DNA revealed that a tiny bit of genetic material on the short arm of the X chromosome had been duplicated. As a result, each of the subjects had a double dose of a gene designated as DSS. This condition resulted in feminization of an otherwise chromosomally normal male fetus (Bardoni et al., 1994).
These findings suggest that a gene (or genes) on the X chromosome helps to push the undifferentiated gonads in a female direction just as the SRY gene helps to start construction of male sex structures. Such observations contradict the long-held belief that the human fetus is inherently female and that, unlike male prenatal differentiation, no gene triggers are necessary for female differentiation.
gonads
the male and female sex glands: ovaries and testes.