Genetics and Gender

The words gene and gender both come from the same Greek word genos, meaning birth, origin, race, species, or class. Genes are the ordered bits of chemical information composed of DNA (deoxyribonucleic acid) that direct all of the bodily processes from the conception and development of an organism to the processes necessary to sustain life. Genes are located on chromosomes in every cell in the body. Human bodies have twenty-three paired chromosomes in cell nuclei. How the two chromosomes in each pair work together defines the kinds of traits (eye color, skin tone, hair type, body shape, metabolism, muscle type) individuals may have and contribute to or even cause potential weaknesses (heart disease, arthritis, cancer) individuals may experience through their lives. There is also DNA in other parts of cells called mitochondria, which are energy-producing "organelles" in each cell.

Genes are passed from parents to children in the gametes (eggs and sperm) that join to make new beings. Although human body cells typically reproduce themselves by a process called mitosis in which the cell divides and replicates its full quota of twenty-three chromosome pairs, germ cells—which are the cells involved in reproduction—are produced by a process called meiosis in which each germ cell receives only one side of each of the cell's twenty-three pairs of chromosomes. In human reproduction, each parent contributes a gamete, or germ cell, produced through meiosis. The mother contributes an egg that is a complete cell with one-half of each of the twenty-three chromosome pairs. The father contributes a sperm that consists of twenty-three half chromosomes contained in a nucleus-like head with a tail. The twenty-three single chromosomes from one gamete then pair up with the twenty-three single chromosomes from the other gamete to form a new full set of twenty-three chromosome pairs.

The twenty-third pair of chromosomes determines an individual's sex (maleness or femaleness). This chromosome comes in two versions: an X chromosome and a Y chromosome. If an individual has two X chromosomes, then, assuming a normal fetal development, that individual will be female with female genitals and reproductive organs (vagina, uterus, ovaries). If an individual has an X chromosome and a Y chromosome, then, assuming a normal fetal development, that individual will be male with male genitals and reproductive organs (prostate, testes).

Because females have only the X chromosome in pair 23, when they produce germ cells through meiosis, their gametes will have only an X chromosome. Because males have both an X and a Y chromosome in that pair, they will produce sperm with either an X chromosome or a Y chromosome. The type of chromosome contained in the sperm, then, will determine the sex of any new individual in combination with the female's X. This means that all Y chromosomes come from fathers. Because the mother's egg is a complete cell, new individuals will inherit mitochondrial DNA only from the mother.

In other species, sex is sometimes determined in ways other than through sex chromosomes. Many insects, such as grasshoppers and roaches, have only an X chromosome. Females have two X chromosomes and males have only one. In birds, butterflies, and some fish, the sex of offspring is determined by the female rather than the male contribution. Females have the sex genotype of ZW, while males have the sex genotype of ZZ. Bees and ants have no sex chromosomes at all. In these species if an egg is fertilized, it becomes a female. If it is unfertilized, it may develop into a male.

SEX CHROMOSOME VARIATIONS

Occasionally during meiosis, there is an error in the splitting of the chromosomes present in future germ cells. Sometimes there is an extra copy of a chromosome so that there are three chromosomes where there should be two (XYY instead of XY, for example). The presence of three chromosomes is called a trisomy, and trisomies often cause problems. Down syndrome, for example, has a trisomy in chromosome 21. Sometimes there is no copy of a sex chromosome, which results in a fetus with only one chromosome 23. Most errors of this sort do not produce viable fetuses. Because trisomies in the sex chromosome tend to cause less severe problems, more fetuses with trisomies and other abnormalities in the sex chromosomes survive.

Having too many or not enough sex chromosomes results in a variety of syndromes and abnormalities. In females, the absence of one X chromosome results in Turner syndrome. Most females with an XO genotype do not survive. Those who do are often mosaics, that is, their genotype varies between XO and XX. This syndrome is relatively rare, occurring in only 1 in 3,000 to 1 in 5,000 births. If these females survive, they tend to be short in stature, with webbed necks, out-turned elbows, high-arched palates, and small jaws. They are prone to thyroid disease, and they may suffer slight mental retardation. They are sterile, as they do not develop normal ovaries nor do they ovulate. They do not develop normal secondary sex characteristics, having small, widely spaced breasts. Girls with Turner syndrome can be treated for its symptoms. Human growth hormone may help them grow taller, and estrogen replacement at puberty will begin menstruation and promote the growth of more normal breasts.

Females who inherit an extra X chromosome are called metafemales or triple-X females. Their genotype is XXX, but can be XXXX or even XXXXX. Metafemales are usually the children of older mothers, and the incidence of this is approximately 1 in 1,000 births. Metafemales do not look much different from XX females. They are fertile, but are generally taller and more slender than their XX counterparts, with longer legs. They may have low intelligence, have learning difficulties, or be perceived as slow learners because of their height when they are young, since they are often perceived to be older than they actually are.

Males with chromosome abnormalities generally suffer either from Klinefelter syndrome or Jacob syndrome (also called XYY syndrome). In Klinefelter syndrome, a male inherits an extra X chromosome, making his genotype XXY or even XXXY, XXXXY, or XY/XXY mosaic (a mixture of genotypes). One of the most common chromosomal abnormalities (1 in 500 to 1 in 1,000 births), Klinefelter syndrome may pass almost unnoticed, or may be expressed as effeminacy accompanied by severe mental retardation, depending on the number of X chromosomes present. Males with Klinefelter syndrome produce very small amounts of testosterone and as a result have small testes and prostate and are nearly sterile. They often have high voices, little body hair, and a more effeminate body shape, and may develop breasts. They are also, like metafemales, taller than average, but they may also be heavier. They have learning difficulties when young, especially with language, but can usually function easily in society, especially if they are treated with testosterone at puberty. Males with Klinefelter syndrome have normal sexual function, can have erections and ejaculate, but may evince less interest in sex. They have a slightly higher likelihood of developing diabetes and osteoporosis.

Although males with XXXXY genotypes have been understood as a variant of Klinefelter syndrome, this genotype is increasingly recognized as its own genetic condition. XXXXY males are characterized by small gonads, micro penis, mental deficiency, speech impairments, hyperextensive joints, low birth weight, and other skeletal anomalies.

Another variant of Klinefelter syndrome now treated as distinct is XXYY syndrome. These boys, with the genotype XXYY, display a range of characteristics including taller than average height, learning disabilities, speech and language impairment, flat feet, scoliosis, delayed sexual development, low testosterone, and developmental delays. Rarer than Klinefelter syndrome, XXYY syndrome occurs in only 1 out every 17,000 births.

Males with an extra Y chromosome have Jacob syndrome, sometimes called XYY syndrome. These boys have an XYY genotype and appear normal. Occurring in between 1 and 900 to 1 in 2,000 births, XYY syndrome boys tend to be taller than average, with high levels of testosterone, acne, and poor coordination. They are fertile and have normal sexual function. There are some claims that XYY males are more disposed to aggressivity and violence. Defense lawyers have attempted to use the genotype as a defense for criminal behavior, but unsuccessfully.

HERMAPHRODITISM AND INTERSEXUALITY

Occasionally, but very rarely, errors in germ cell production result in a fetus with both XX and XY genotypes simultaneously. The presence of both genotypes in the body causes the development of both male and female sex organs. Externally, the fetus may develop a penis, but testes are usually underdeveloped and undescended. Often the external genitalia are ambiguous, with fused labia, a clitoris that looks like a penis, a penis that is underdeveloped, or a vaginal opening without a complete vagina. Ovaries, vagina, and uterus will develop internally but may consist of an ovary on one side and a testis on the other, or a testis or ovary on one side and an ovo-testis (a mixture of ovary and testicle) on the other. Until recently most doctors thought that the external genitalia of intersexuals needed to conform to one sex or another. In addition to surgeries necessary to correct life-threatening conditions, doctors often performed more cosmetic corrections that would make hermaphrodites conform to one sex.

Genes other than those comprising the twenty-third pair of chromosomes can affect the sexual development of embryos, making them appear to be a sex different than that indicated by their genotype. Androgen insensitivity syndrome (AIS), for example, is a condition in which XY fetuses that have begun at the eighth week to develop testes, do not continue to develop normal male genitalia because their body tissues cannot use the androgens their testes produce. A genetically male individual will have the appearance of a female. AIS is caused by a faulty androgen receptor gene located on the X chromosome. The syndrome is thus passed onto genetically male children by the mother.

There are two forms of AIS: complete and partial. In complete AIS the body's tissues are completely insensitive to androgens, and the body develops as a female without internal sexual organs. In partial AIS some tissues are sensitive to androgen, but in varying degrees. Individuals with partial AIS have a range of external genitalia from normal male external genitalia and infertility to genitals that appear to be female with an enlarged clitoris or even genitals that appear to be completely female. AIS comes with a higher risk of cancer affecting especially the unformed testicular tissue still in the body. This tissue is generally removed at an early age to prevent further problems.

Another genetic disorder, congenital adrenal hyperplasia (CAH), encompasses several different conditions in which the steroid cortisol is not produced, causing the overproduction of other steroids. Female fetuses with this recessive trait produce too many steroids and often develop genitals that appear to be male, with large clitorises or even penises. Such females are often mistaken for boys at birth. Girls with CAH often have low rates of fertility.

SEX-LINKED DISORDERS

The X and Y chromosomes of pair 23 are not of equal size, though together they are responsible for most sex-based characteristics. The X chromosome is much larger than the Y and has more genes. The genes located on chromosome 23 manage the development of genitals and gonads, the timing of puberty, the production of sex hormones, and the appearance of secondary sex characteristics. Their management of these processes, however, does not come only from direct instruction, but also as an effect of being or not being paired with another gene on the pairing chromosome (X with X, X with Y). Because the Y chromosome is much shorter than the X chromosome, portions of the X chromosome have no correlative gene on the Y. This means that certain traits, such as color blindness, hemophilia, Duchenne muscular dystrophy, and fragile X syndrome—the genes for which are located on the X chromosome—have no corresponding version on the shorter Y. Thus, these traits are exhibited primarily by male children and are inherited from the mother. In some disorders, females are also affected, but often not as severely because they have another copy of the X gene that may still function. The X chromosome is linked to more than three hundred diseases, more than any other chromosome, including not only sex-linked conditions, but also disorders such as cleft palate and chronic granulomatous disease; it may also contain the genes for longevity.

Color blindness occurs because the gene for normal color vision occurs on the X chromosome. If XN stands for the gene for normal vision and Xn stands for the gene for color blindness, then a male with the genotype XnY would be color blind, while a male with the genotype XNY would have normal vision. A female with the genotype XNXn would have normal vision but would be a carrier of the color-blindness trait. Females can inherit color blindness, but only if their fathers are color blind and their mothers are carriers. The genotype of a color-blind female would be XnXn.

People with hemophilia, called hemophiliacs, lack the blood-clotting factor VIII. As with color blindness, women carry the recessive gene on the X chromosome and so are rarely affected by the condition themselves, unless they inherit two versions of the gene, one from a hemophiliac father. Queen Victoria was a carrier of this recessive gene, so her sons had a 50/50 chance of having hemophilia. One of Victoria's sons and three of her grandsons were hemophiliacs, and two daughters were carriers.

Duchenne muscular dystrophy is a condition in which males lack the gene for producing a key muscle protein called dystrophin. This gene, too, is located on the X chromosome. Males with Duchenne muscular dystrophy are afflicted by a weakening of the muscles and a loss of coordination, dying in early adulthood. This condition occurs in approximately in 1 in every 3,500 births.

Fragile X syndrome involves a mutation in a gene for producing a protein necessary for proper cell, especially brain cell, development and functioning. This gene, located on the X chromosome, generally affects intelligence in male children, though females with one copy of the gene will also have reduced mental capacity. Those with fragile X syndrome often have large ears, long faces, and some problems with emotion and behavior. The mutation causing fragile X syndrome occurs in the number of repeated sequences that tell a gene when to turn on and turn off. Fragile X syndrome involves a larger than normal number of repeats, which slows down or prevents the operation of the gene.

GENES AND GENDERED BEHAVIOR

Some researchers believe that genes influence the development of the brain differently in females and males. They attribute certain traits that have come to be understood as gender stereotypes to differences in brain structure and function. Thus, for example, having more precocious linguistic abilities or having the ability to gauge spatial relations may not be an effect of children being nurtured as girls or boys, but may be hard-wired into the structures of differently sexed brains. Some of the evidence that suggests that genderings are genetic comes from the experiences of intersexed babies who are reassigned a sex opposite to that of their genotype. As these children grow older, they often wish to become the other sex, even though they have most often never been told about their genotypes.

Within the last ten years research on the sex chromosomes has shown that there is a substantial difference in the way as many as three hundred genes on the X chromosome are activated between males and females. Scientists suggest that males and females may differ by as much as 2 percent of their genome. This difference is greater than the difference between humans and chimpanzees.

Other researchers attribute differences in brain function and sex identifications to the actions of hormones during fetal development. These researchers point to the ways genetic males and females will manifest abilities linked to the other sex and will often prefer cross-gendered behaviors. Some of the evidence that suggests that hormones are also influential comes from the range of behaviors and identifications genetically and somatically normal people seem to display.

Ideas about the genetic or hormonal basis for gender differences contravene earlier ideas that gender was learned through culture or nurture. The early-twenty-first-century consensus seems to be that sex and gender are formed through a complex interaction of genes, hormones, and nurture that generally aligns genotypes with bodies and with behaviors that comply with normative gender concepts.

DIFFERENCES WITHIN SEXES

Genes do not, however, affect only differences between the sexes. They also seem to produce a range of differences within both women and men. The large number of variations in the distribution of genes on the X chromosome produces a wide range of differences among women themselves. Researchers have also discovered that, although generally in women one copy of the genes on the X chromosome is turned off, in some women more genes are still activated on both copies, producing even more variation. These variations may account for differences in the ways women react to drugs or their vulnerability to disease. They may also account for differences in preferences, activities, and degrees of feminization, although these qualities are also determined at least in part by the cultural models and opportunities available.

Both Y chromosomes and the mitochondrial DNA passed from mothers to children also enable scientists to trace different groups of people. Y chromosomes, passed only from fathers to sons, change little, and these changes tend to accompany surnames. Only the gradual accumulation of mutations differentiates separate groups of males. By looking at Y chromosomes, researchers have been able to trace the movements of groups of people across continents and through history. They have also posited the existence of a "Y-chromosomal Adam" as a forefather of most living beings.

The mitochondrial DNA passed on by mothers also is only altered by the slow accumulation of mutations. Some researchers have posited the existence of an originary mother figure, called "Mitochondrial Eve" or sometimes "African Eve," supporting the idea that all humans emerged from a single African ancestor. Mitochondrial Eve is much older than Y-chromosomal Adam. Both figures are disputed hypotheses.

see also Gender Identity.

BIBLIOGRAPHY

Barash, David P., and Judith Eve Lipton. 2002. Gender Gap: The Biology of Male-Female Differences. New Brunswick, NJ: Transaction Publishers.

Fausto-Sterling, Anne. 2000. Sexing the Body: Gender Politics and the Construction of Sexuality. New York: Basic.

Hubbard, Ruth. 1979. Genes and Gender II: Pitfalls in Research on Sex and Gender. New York: Gordian Press.

Kaplan, Gisela, and Lesley J. Rogers. 2003. Gene Worship: Moving Beyond the Nature/Nurture Debate over Genes, Brain, and Gender. New York: Other Press.

Ridley, Matt. 1999. Genome: The Autobiography of a Species in Twenty-three Chapters. New York: HarperCollins.

                                                   Judith Roof