Human beings have a biological urge to reproduce and so are in some sense “designed” to be sexual beings; any species that does not have good reproductive equipment and a strong desire to use it will not last very long. Simpler organisms, such as amoebas, simply split in two, creating a pair genetically identical to the parent amoeba. More complex organisms, however, reproduce through sexual reproduction, in which two parents each donate a gamete (GAM-meet), or germ cell, which combine to create a new organism.
The tiny germ cells from the male (sperm) and the much larger but also microscopic cell from the female (egg, or ovum) each contain half of the new person’s genes and determine his or her sex, hair and eye color, general body shape, the likely age at which he or she will reach puberty, and literally millions of other aspects of the new person’s physiology, development, and emotional nature. The genes direct the development of the genitals and the reproductive organs and set the biological clock running to trigger puberty and female menopause or male andropause. We will discuss both of these in the next two chapters.
2Today, the Intersex Society of North America (ISNA) is working to end the shame, secrecy, and unwanted genital surgery for people who experience such a trauma or who are born with ambiguous genitalia (http://www. isna. org).
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Most cells in the human body contain 46 chromosomes: 23 inherited from the mother and 23 from the father, arranged in 23 pairs. Twenty-two of the pairs look almost identical and are referred to as autosomes; the exception is the 23rd pair, the sex chromosomes. The two sex chromosomes, which determine whether a person is male or female, are made up of an X chromosome donated by the mother through the ovum and either an X or a Y chromosome donated by the father’s sperm. In normal development, if the male contributes an X chromosome, the child will be female (XX); if he contributes a Y, the child will be male (XY).
All the cells of the body (somatic cells), except gametes, contain all 23 pairs of chromosomes (46 total) and are called diploid (meaning double). But if a merging sperm and egg also had 23 pairs each, they would create a child with 46 pairs, which is too many (remember that most cells contain only 23 pairs of chromosomes). So gametes are haploid, meaning they contain half the number of chromosomes (23) of a somatic cell (46). During fertilization, a haploid sperm and a haploid egg join to produce a diploid zygote (ZIE-goat) containing 46 chromosomes, half from each parent. The zygote can now undergo mitosis, reproducing its 46 chromosomes as it grows.
The 46 chromosomes are threadlike bodies made up of somewhere between 20,000 to 25,000 genes, each of which contains deoxyribonucleic (dee-OCK-see-rye-bow-new- KLEE-ik) acid (DNA; Human Genome Project, 2003). DNA acts as a blueprint for how every cell in the organism will develop. At first, the zygote reproduces exact copies of itself. Soon, however, the cells begin a process of differentiation. Differentiation is one of the great mysteries of human biology—suddenly, identical cells begin splitting into liver cells, brain cells, skin cells, and all the thousands of different kinds of cells in the body. The DNA determines the order in which cells differentiate, and a cell’s position may de-
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autosome
Any chromosome that is not a sex chromosome.
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sex chromosomes
Rod-shaped bodies in the nucleus of a cell at the time of cell division that contain information about whether the fetus will become male or female.
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fertilization
Occurs when a haploid sperm and a haploid egg join to produce a diploid zygote, containing 46 chromosomes.
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zygote
The single cell resulting from the union of sperm and egg cells.
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mitosis
The division of the nucleus of a cell into two new cells such that each new daughter cell has the same number and kind of chromosomes as the original parent.
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deoxyribonucleic acid (DNA)
A nucleic acid in the shape of a double helix, in which all genetic information in the organism is encoded.
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Question: Does the father’s sperm really determine the sex of the child?
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Yes, it is the sperm that determines the sex of the child, but the woman’s body does have a role to play; there are differences between X and Y sperm (X’s are heavier and slower but live longer; Y’s are faster but die more quickly), and a woman’s vaginal environment or ovulation cycle may favor one or the other. But the sex of the child does depend on whether an X chromosome sperm or a Y chromosome sperm, donated by the father, joins with the ovum (which is always an X). The irony is that for many years, in many cultures, men routinely blamed and even divorced women who did not produce a child of a certain sex (usually a boy), when in fact the man’s sperm had much more to do with it.
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"Scientists stained the sperm and marked them through a laser, so the X
chromosomes may be separated from the Y chromosomes."—Choosing Your Child’s Gender
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termine to some degree what type of cell it will become. Researchers in evolutionary developmental biology explore how and when cells differentiate.
Whether the zygote will develop into a male or female is determined at the moment of conception, and part of the process of differentiation includes the development of our sexual characteristics. If sexual differentiation proceeds without a problem, the zygote will develop into a fetus with typically male or typically female sexual characteristics. However, a variety of things can happen during development that can later influence the person’s own sense of being either male or female.
