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Sex and Genetics

Many people have asked questions about what makes a man a man and a woman a woman. For many people, this is a simple answer: if someone has a penis then they are a male and if that someone has a vagina then they are a female. Modern medicine has shown us that answers such as this are a bit too simplistic.

A person's physical characteristics are a result of a combination of environmental factors and that person's ancestry. It is common knowledge that a mother's health influences the physical development of an unborn child in their womb. This is why pregnant women are advised not to drink alcohol, smoke, take recreational drugs, and are advised to eat properly and take pre-natal vitamins. There is a wealth of science that proves heredity influences the development of a person's body. As early as 1865, Gregor Mendel, an Augustinian monk who is known as the "father of modern genetics," shown that inheritance is based on pairs of factors and these factors occur in pairs in each parent. These factors were later identified as chromosomes.

This combination of environment and heredity influences just about everything for a given person. This influence also extends to a person's sex.

Putting aside environmental factors for now, let's consider how physical sex is normally determined through heredity. A baby is a result of a sperm cell and an egg cell. Each cell type is produced by the parent's bodies from "normal body cells." Except for sperm and eggs, each cell in the body has 23 pairs of chromosomes. Each chromosome in a given pair may or may not have the same genetic instructions. For example, in one pair of chromosomes there may be a gene on one chromosome for blue eyes and a gene on the other chromosome for brown eyes.

When a body cell is transformed into sperm, that cell is divided in half to produced sperm cells. Each sperm cell receives one chromosome from each pair of chromosomes found in the original body cell. The same division of chromosomes occurs when a body cell is transformed into an egg. In the above example, this would result in one sperm containing the gene for blue eyes and the other sperm containing the gene for brown eyes.

Sex Chromosomes From Parents Just like eye color, there are genes that indicate if a sperm will produce a male or produce a female. These genes are located on a pair of chromosome called the "sex chromosomes." When viewed under a microscope, these chromosomes come in two shapes. Since one looks the letter "X" and the other like the letter "Y," they are known as the "X chromosome" and the "Y chromosome," respectively.

On the Y chromosome, at a particular spot which is not found on the X chromosome, there is a gene called the "sex-determining region of the Y" (SRY), also called the "maleness gene." It is this gene that is responsible for turning an embryo into a male. Normally, if a person does not have this Y chromosome then they do not have the "maleness gene" and therefore will not develop as a male.

Normal healthy women will have two X chromosomes and no Y chromosomes. Without Y chromosomes, a woman can produce only eggs that contain X chromosomes. Men will have at least one Y chromosome. In fact, a normal man will have exactly one Y chromosome. This is because his mother can only provide an X chromosome so the Y chromosome must come from the father.

In other words, when the sperm with an X chromosome joins with an egg, a female will result. If a sperm with an Y chromosome unites with an egg, a male results.

Of course, sexual development does not end at this point. In the weeks that follow conception, the "maleness gene," if present, causes testes to develop and testosterone produced. It is the presence of testosterone that causes the development of other male sex organs, such as the penis and prostate. Without testosterone, female sex organs develop. It is believed that testosterone levels effect the development of certain structures in the brain that differ somewhat between males and females. Also, at puberty, secondary sex characteristics develop.

In most circumstances, this chain of events stemming from the presence or absence of the "maleness gene" results in a normal, healthy adult that is able to have children themselves. There are many opportunities for events to occur in other ways, which may produce any number of results, including intersex (and possibly other transgender) individuals.

As complicated as genetics may be, gender may be just as complicated. Not only is gender a result of biology, it is also a cultural phenomenon. How much of one's gender is because of their genes and hormones and how much of it nurture? Are we more than the sum of our chromosomes? How could any one person really know what is another person's gender? If so, how could I tell you that you were right or if you were wrong?