the microscope. Scientists were seeing that tissues are made up of cells and that new cells come from the division of old cells. As two new cells form, the old cell produces two copies of everything so that each new cell has exactly what the old cell had. Mitosis is the whole process by which one cell becomes two. One particular area of the cell, which looks like a cell within the cell, was especially intriguing to scientists. This area is called the nucleus of the cell. As two new cells are being formed, some substances in the nucleus double and separate so that each new cell gets a complete set of these substances, called chromosomes. Over the years, scientists discovered that each plant and animal has a set of chromosomes, but the numbers of chromosomes may differ between species. For instance, humans have 23 pairs, or 46, chromosomes, while chimpanzees have 24 pairs, or 48, chromosomes. (But chimpanzee chromosomes look more like human chromosomes than ape chromosomes, so don’t be thinking that you’re so smart.) Examining the division of egg cells and sperm cells (the so-called germ cells), scientists discovered that each of these cells contains only half the normal number of chromosomes. In humans, for example, each egg cell and each sperm cell has one set of 23 chromosomes (while other human cells have 46 chromosomes). When these cells divide to form more sperm or egg cells through a process called meiosis, the result is again 23 chromosomes per cell. When the egg and the sperm join together in fertilization, the combination, called a zygote, has the normal number of 46 (23 pairs) chromosomes. When a zygote forms, one set of its chromosomes comes from the female, and one set of chromosomes comes from the male; these sets pair up two by two. The members of each chromosome pair are called homologous chromosomes. As is always the case, the rule that all chromosome pairs have identical sets of genes has an exception. But like the French say, vive le difference. Loosely translated, that means “thank goodness for this particular set of chromosomes.” I’m referring to the sex chromosomes that determine whether you’re a boy or a girl. All other pairs of chromosomes have matched genes; if a gene exists for a given characteristic on one of the chromosomes of the pair, the other chromosome has a gene for that same characteristic. A female has two matched sex chromosomes (called X chromosomes), and a male has two different sex chromosomes (called an X chromosome and a Y chromosome).
Mapping traits with genes and chromosomes