ZO sex-determination system

The ZO Way to Decide Sex

Some tiny animals, like caddisflies and a few moths, have a special way to decide if a baby is male or female. This way is called the ZO sex-determination system.

In these animals, there is a tiny part called a chromosome named Z. Males have two Z chromosomes, so they are called ZZ. Females only have one Z chromosome, so they are called ZO.

This ZO way comes from the old ancestors of Lepidoptera, which includes beautiful butterflies and many moths. Even though many of these insects now use a different way called the ZW system, some still use the ZO way. This shows how animals can change and find new ways to grow and live.