Sex cells—eggs in females and sperm in males—can fuse to form a fertilized cell, or zygote. But how can two cells be fused without making a cell with too many chromosomes? Your skin, bone and brain cells all have two sets each of 23 pairs of chromosomes. Each set of chromosomes is called a homologous pair and in each homologous pair there is one chromosome that came from the mother and one from the father. That is, you have 23 homologous pairs of chromosomes in almost all of the cells in your body, resulting in 46 total chromosomes. In humans, if a cell with 23 pairs of chromosomes fused with another cell with 23 pairs of chromosomes, the new cell would have 46 pairs of chromosomes and 4 copies of each type of chromosome. For organisms that are diploid (meaning they have two sets of each chromosome) having more than two sets of chromosomes often results in problems with cell function. Instead, sex cells (sperm and eggs) have only half the number of chromosomes of a non-sex (or autosomal), cell. To achieve half the number of chromosomes, the germ cells (the cells that form sperm and eggs) undergo a process called meiosis. In human meiosis, each of the resulting cells has only one of each of your 23 chromosome pairs instead of 23 complete pairs of chromosomes. That is, each product of meiosis (egg or sperm) has only 23 chromosomes; egg and sperm cells do not contain homologous pairs.
To represent this with simple graphics, we represent meiosis here in an organism that has only two pairs of chromosomes. Keep in mind that for humans, there are actually 23 chromosome pairs undergoing these processes. Other organisms have different numbers of chromosomes: fruit flies have 4 pairs, or 8 total chromosomes; chickens have 39 pairs, or 78 total chromosomes; and chimpanzees have 22 pairs, or 44 total chromosomes.
A critical concept in our discussion of cell division and inheritance is that of ploidy, or the number of complete sets of chromosomes in a cell. Cells can be monoploid (1 set), diploid (2 sets), triploid (3 sets), and so on. In much of this chapter, we discuss humans; humans are diploid organisms, with two complete sets of chromosomes—one set from the father and one set from the mother–in almost all of their cells. The exceptions are the gametes, which are haploid. For diploid organisms such as humans, chickens, fruit flies, and chimpanzees, the haploid number is “1,” and refers to the fact that their sperm and egg cells have only one complete set of chromosomes. You’ll note that the term n is used to describe an organism’s haploid state; in diploid organisms, all the non-sex cells, or autosomal cells, are 2n.
To complicate terminology further, only roughly half of humans have 23 homologous pairs. Women, who are genetically characterized by the presence of two X chromosomes, have 22 non-sex chromosomes and a pair of Xs. These X chromosomes have the same genes at the same locations and are thus homologues like the other 22 pairs of chromosomes. However, human males are typically characterized by having an X chromosome and a Y chromosome. These chromosomes are different in size, shape, and the genes they carry. Therefore, they are not homologues. Consequently, human males have 22 homologous pairs and one non-homologous pair of sex chromosomes—XY. It is typical for us to refer to humans—males and females—as having 23 pairs of homologous chromosomes, but now you know: it’s actually not quite that simple!]