Sexual reproduction depends in part on meiosis (my OH sis)—a type of cell division that produces four cells, each with half the number of chromosomes as the parent cell. In animals, meiosis occurs in the sex organs—the testes in males and the ovaries in females. To understand the process of meiosis, you first need to learn a little more about chromosomes and their role inthe life cycles of sexually reproducing organisms.
If you examined a number of cells from any individual organism, you would see that almost all the cells have the same number and types of chromosomes. Likewise, cells from different male or female individuals of a single species have the same number and types of chromosomes. For example, a typical human body cell has 46 chromosomes. A display ofthe 46 chromosomes of an individual is called a karyotype (KAR ee uh type). Each chromosome has a twin that resembles it in size and shape. You inherit one chromosome of each pair from your mother and the other from your father. The two chromosomes of each matching pair are called homologous chromosomes (Figure 9-13).
Karyotype displays are made when chromosomes are visible duringmetaphase; therefore, each chromosome actually consists of two chromatids. In a true karyotype you would not be able to distinguish the sister chromatids. This illustration, however, shows how a homologous pair would look if you could distinguish the sister chromatids on each chromosome.
Each homologous chromosome in a pair carries the same sequence of genes controlling the same inheritedcharacteristics. For example, if a gene influencing eye color is located at a particular place on one chromosome, then the homologous chromosome also has a gene for eye color in the same place. However, the two genes may be slightly different versions. One chromosome might have the form of the gene for brown eye color, while the homologous chromosome might have another form for blue eye color. Note howthis comparison is different from that of the sister chromatids you read about earlier in this chapter. Sister chromatids are duplicated copies of a single chromosome that are attached to each other and are identical—both chromatids contain exactly the same forms of each gene.
Altogether, humans have 23 homologous pairs of chromosomes. For the karyotype of a human female, the 46 chromosomes fallneatly into 23 homologous pairs. But in males, the two chromosomes of one pair do not look alike. This 23rd pair of chromosomes, called the sex chromosomes, determines the person's sex. The sex chromosomes occur in two forms, called X and Y. Like all mammals, human males have one X chromosome and one Y chromosome, while females have two X chromosomes. Only small parts of the X and Y chromosomesare homologous. Most of the genes carried on the X chromosome do not have counterparts on the tiny Y, and the Y has genes that are lacking on the X.
Diploid and Haploid Cells
Having two sets of chromosomes, one inherited from each parent, is a key factor in the life cycles of all sexually reproducing organisms. You can follow the human chromosomes through the human life cycle in Figure 9-15.Figure 9-15
In the human life cycle a haploid egg and sperm fuse and form a diploid zygote. Mitosis produces an embryo with numerous cells that continue to multiply and develop.
Almost all human cells are diploid; that is, they contain two homologous sets of chromosomes. The total number of chromosomes, 46 in humans, is referred to as the diploid number (abbreviated 2n, as in 2n = 46). Theexceptions are egg and sperm cells, known as sex cells, or gametes (GAM eets). Each gamete has a single set of chromosomes, one from each homologous pair. A cell with a single set of chromosomes is called a haploid cell. For humans, the haploid number (abbreviated n) is 23. These haploid cells are produced through the process of meiosis, which you will read more about later in this section.