6.11: Male and female gametes are produced in slightly different ways.

No matter how small or large they are, no matter whether they’re plants or animals, for all sexually reproducing organisms there is just one way to distinguish males from females. Regardless of the species, the defining feature is always the same (and if you’re thinking of anything visible to the naked eye, you’re wrong). When there are two sexes—as there are in nearly every sexually reproducing animal species—the females are the sex that produces the larger gamete, and the males produce the smaller, more motile, gamete (FIGURE 6-25). Because all gametes in animals are produced through meiosis, it is a slight variation in how meiosis works in males and females that leads to this difference.

The female gamete is larger than the male gamete because it has more cytoplasm. During the production of sperm, the two divisions occur just as described in Section 6-10, resulting in four evenly sized cells that become sperm. During the production of eggs, things are a bit different.

The first division occurs just as described in Section 6-10, except that in telophase I, as the cell divides, the genetic material is evenly divided, but nearly all of the cytoplasm goes to one of the cells and almost none goes to the other. The smaller cell is called a polar body and degrades almost immediately in most animals (although it sometimes goes through a second division, after which, in most animals, both of those polar bodies degrade). Then, in the second meiotic division of the larger cell, there is again an unequal division of cytoplasm.

As in the first division, one of the new cells gets nearly all of the cytoplasm and the other gets almost none, forming another polar body. The net result of meiosis in the production of eggs is one large egg with lots of cytoplasm and two or three small polar bodies with very little cytoplasm that degrade and never function as gametes (FIGURE 6-26).

Ultimately, whether eggs or sperm are produced, each gamete ends up with just one copy of each chromosome. That way, the fertilized egg that results from the fusion of sperm and egg carries two complete sets of chromosomes, so the developing individual will be diploid. The extra cytoplasm carried by the egg contains a large supply of nutrients and other chemical resources to help with the initial development of the organism following fertilization.