In extraordinary cases, some animals have each chromosome in very particular physical regions. Meet the gynandromorphs — parts of their bodies are male...and the other parts are female.
The chromosomes of an animal determine its sex. But some animals have sex chromosomes that aren't expressed. Some have chromosomes that are expressed internally, whereas a different set are expressed externally. Nature has many ways of combining genes, and some of these combinations evince a distinctly binary way of looking at the world. Such is the case with a genetic condition known as gynandromorphism.
Gynandromorphism is unknown in humans, but it's sometimes expressed in birds, crustaceans, arachnids, and insects. In insects, it's possible that this condition is caused when two sperm enter the egg. One fuses with the nucleus and creates a female insect, the other continues on alone and creates a male insect — both are at work in one body. In birds, gynandromorphism may be caused by an inability of the sex chromosomes to separate when the newly-fertilized cell first splits in two.
Gynandromorphs can develop many different ways. Some can be a patchwork of male and female body parts — some birds even show male and female sections of the same feather. But bilateral gynandromorphism is something special.
In perfect bilateral gynandromorphism, the animal's body is perfectly divided down the center. One side is male and the other side is female. Butterflies have two different colored wings. Chickens have different plumage on each side of the body. In some animals, such as insects, the size of each side of the body is markedly different.
This not only gives clues on how gynandromorphs develop, but how bodies develop in general. It seems that the first division of the fertilized cell could give rise to each side of an animal's body. Left is left and right is right, even from the beginning.