6.14–6.15: There are sex differences in the chromosomes.

XX and XY: only one of the 23 chromosome pairs determines sex in humans.

In humans, the sex of a baby is determined by its father. The sequence of events involved in sex determination is instigated by one special pair of chromosomes, the sex chromosomes, which carry information that directs a growing fetus to develop as a male or as a female.

Let’s take a closer look at the human sex chromosomes. We noted that there are 23 pairs of chromosomes in every somatic cell. These can be divided into two different types: 1 pair of sex chromosomes and 22 pairs of non-sex chromosomes. The human sex chromosomes are called the X and Y chromosomes (FIGURE 6-31).

Figure 6.31: X and Y: the human sex chromosomes.

Figure 6.32: Sex determination in humans. The sex of offspring is determined by the father.

How do the X and Y chromosomes differ from the other chromosomes? All of the genetic information is stored on the chromosomes in all the cells of an organism’s body. But most of this information is not sex specific—that is, if you are building an eye or a neuron or a skin cell or a digestive enzyme, it doesn’t matter whether it is for a male or for a female; the instructions are the same for both sexes. Some genetic information, however, instructs the body to develop into one sex or the other. That information is found on the sex chromosomes.

An individual has two copies of all the non-sex chromosomes (called autosomes). One copy is inherited from the mother, one from the father. Individuals also have two copies of the sex chromosomes, but not always two copies of the same kind. Males have one copy of the X chromosome and one copy of the Y chromosome. Females, on the other hand, don’t have a Y chromosome but instead have two copies of the X chromosome.

So how does the father determine the sex of the baby? During meiosis in females, the gametes that are produced carry only one copy of each chromosome. This is true for the sex chromosomes, too. So from the two copies of the X chromosome carried by females, half of the gametes end up with a copy of one of those X chromosomes while the rest of the gametes inherit the other X chromosome. Thus, every egg has an X for its one sex chromosome. During meiosis in males, the sperm that are produced also carry one copy of each chromosome, including the sex chromosomes, but in this case, half of the sperm produced inherit the X chromosome and the other half inherit the Y chromosome. At fertilization, an egg bearing a single X chromosome (and one copy of all the non-sex chromosomes) is fertilized by a sperm bearing one copy of all the non-sex chromosomes and either an X chromosome or a Y chromosome. When the sperm carries an X, the baby will have two X chromosomes and will develop as a female. When the sperm carries a Y, the baby will have an X and a Y and so will develop as a male (FIGURE 6-32).

Given the distribution of X and Y chromosomes into males and females, it must be true that no essential genetic information is carried on the Y chromosome. Why? Because females don’t have a Y chromosome in any of their cells, yet they are able to develop and live normal, healthy lives. For this reason, we know that nothing on the Y chromosome is absolutely necessary for the development of a normally functioning human.

Physically, the X and Y chromosomes look very different from each other. The X chromosome is relatively large and carries a great deal of genetic information relating to a large number of non-sex-related traits. The Y chromosome is tiny and carries genetic information about only a very small number of traits. The genetic instructions on the Y chromosome instruct the fetal gonads to develop as testes rather than ovaries. Once this is done, very little additional genetic input from the Y chromosome is necessary. Instead, the hormones produced by the testes or ovaries generally direct the rest of the body to develop as a male or female. In the next sections we investigate some of the ramifications of males having two different sex chromosomes and look at some of the systems for sex determination that have evolved in other groups of organisms.