A Matter of Sex

Sex had to come into this somewhere. After all, the subject is biology.

To a certain extent, most everyone knows the story of sex determination — you know, the X and Y stuff. However, it turns out that there's more than one way to create two different genders — not all kinds of organisms do it the same way.

The familiar pattern of X and Y chromosomes is characteristic of all mammals. Certain insects have pretty much the same kind of sex determining system, though the actual way it works is a bit different than the way mammals do it. In the X and Y system, there are two kinds of sex chromosomes, the X chromosome and the Y chromosome, which is very small. Females have two X chromosomes (XX) and males have one X chromosome and one Y chromosome (XY). Mammals and fruit flies use this system. Grasshoppers take this pattern to an extreme. Female grasshoppers have two X chromosomes; males have one X chromosome with no partner at all.

Birds have a similar system, but it works in the opposite direction. Their sex chromosomes are called Z and W. Male birds have two identical sex chromosomes (ZZ) and female birds have one Z and one W (ZW).

In many reptiles, sex determination has nothing to do with chromosomes at all. It has to do with temperature.

Alligator hatching from egg.

For instance, alligator eggs which incubate at higher temperatures hatch out male baby alligators. The cooler ones hatch out females.

Then there are fish. In many fish species, an individual is male when it is small, but when it grows bigger, it changes into a female. In many cases, this change is tied to behavior. In a lot of fish species where the fish normally swim in schools, there will be one female and several males in a school. As long as the female is alive and well, the others stay smaller and male. If the female dies, then at least one of the males will grow larger and change gender. It sounds weird, but not to the fish.

Queen bee and drone

Finally, we have the case of honey bees. In honey bees, almost all of the eggs the queen lays are fertilized.

Every fertilized egg develops into a female bee (mostly workers). When mating season comes around, a few of the eggs are left unfertilized. These develop into males — who are haploid, rather than diploid. A few of the fertilized eggs are fed a special food, and they develop into new queens, who are the only fertile females in the hive. So fertile females have to be diploid and fed special "royal jelly" and males have to be haploid. Actually, it turns out that if you force a hive to inbreed for a number of generations, which makes the population homozygous for more and more genes, a few of the fertilized eggs will develop into diploid males. It turns out that bees, instead of having a pair of sex chromosomes, have a bunch of sex determination genes scattered all over the chromosomes. If a bee is heterozygous for just one of them, the bee will be female. Normally, the only way to make sure that a bee won't be heterozygous at any of the sex determining genes is make the bee haploid — leave the egg unfertilized. Now that's weird.

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Updated 25 September 2004