6.15: The sex of offspring is determined in a variety of ways in non-human species.

For something as fundamental to a species as sex determination, you might imagine that one method evolved and all species use it. The world is more diverse than that. In most plants there aren’t even distinct males and females. In corn, for example, every individual produces both male and female gametes. All earthworms and garden snails are also capable of producing both male and female gametes. Such organisms are called hermaphrodites, because male and female gametes are produced by a single individual. But even among the species with separate males and females there are several different methods of sex determination. Humans that have both an X and a Y chromosome are male, and those that have two X chromosomes are female. This sex determination pattern is common among eukaryotes and is seen in all mammals. Sex determination differs in other organisms, including some birds, insects, and reptiles (FIGURE 6-33).

Birds In birds (as well as in some fish and butterfly species), the mother determines the sex in much the same way that fathers determine sex in humans. Females have one copy of two different sex chromosomes, called the Z and the W chromosomes, while males have only one type, carrying two copies of the Z chromosome. Consequently, the sex of bird offspring is determined by the female rather than the male.

Ants, Bees, and Wasps In these insects, sex is determined by the number of chromosome sets an individual possesses. Males are haploid, having a single set of chromosomes, and females are diploid, carrying two sets of chromosomes. In this method, females produce haploid eggs by meiosis. They mate with males and store the sperm in a sac. As each egg is produced, the female can fertilize it with sperm she has stored, in which case the offspring will be diploid and female. Alternatively, she can lay the unfertilized egg, which develops into a haploid, male individual. Just think, in these species, males don’t have a father, yet they do have a grandfather!

Turtles In some species, sex determination is controlled by the environment rather than by the number or types of chromosomes an organism has. In most turtles, for example, offspring’s sex is determined by the temperature at which the eggs are kept. Eggs kept relatively hot during incubation become females, while eggs incubated at cooler temperatures become males. The sex of some lizards and crocodiles is also determined this way.