6.12: Crossing over and meiosis are important sources of variation.

Genetically speaking, there are two ways to create unique individuals. The obvious way is for an organism to carry an allele that is not present in any other individuals. Alternatively—and equally successful in creating uniqueness—an individual can carry a collection of alleles, no single one of which is unique, that has never before occurred in another individual. Both types of novelty introduce important variation into a population of organisms. The process of crossing over, or genetic recombination (FIGURE 6-27), which occurs during prophase I in meiosis, creates a significant amount of the second type of variation.

As we saw in Section 6-10, during the first prophase of meiosis, the sister chromatids of homologous chromosomes all come together. Let’s review exactly what it means. Let’s look at just one of your homologous pairs of chromosomes, the homologous pair of chromosome 15. When we refer to the “homologous pair” of chromosome 15, remember that this pair includes two copies of chromosome 15: one copy from your mother (which you inherited from the egg that was fertilized to create you) and one copy from your father (which you inherited from the sperm that fertilized the egg). Each chromosome in the pair carries the same genes, but because they came from different people, they don’t necessarily have the same alleles.

Once the sister chromatids of the homologous chromosome pairs line up (so that there are now four chromatids in two pairs lying very close together), regions that are close together can swap segments. A piece of one of the maternal chromatids—perhaps including the first 100 genes on the strand of DNA—may swap places with the same segment in a paternal chromatid. Elsewhere, a stretch of 20 genes in the middle may be swapped from the other maternal chromatid with one of the paternal chromatids. The points at which chromatids exchange genetic material during recombination are called chiasmata (sing. chiasma). Every time a swap of DNA segments occurs, an identical amount of genetic material is exchanged, so all four chromatids still contain the complete set of genes that make up the chromosome. The combination of alleles on each chromatid, though, is now different.

Suppose there are genes relating to eye color, hair color, and height on a particular chromosome. After crossing over, the linear strands still have instructions for all of those traits. But whereas one chromatid previously may have had instructions for brown eyes, brown hair, and short height, it may now have some differences—perhaps brown eyes, blond hair, and tall height. All of the alleles from your parents are still carried on one DNA molecule or another. But the combination of traits that are linked together on a single chromatid is new. And when a gamete, let’s say it’s an egg, carrying a new combination of alleles is fertilized by a sperm, the developing individual will carry a completely novel set of alleles. And so without creating new versions of any traits (such as yellow eyes or purple hair), crossing over still creates gametes with collections of alleles that may never have existed together. In Chapter 8, we’ll see that this variation is tremendously important for evolution.