Principles of Genetics and Breeding

4.1.7.4 XO – system & ZO - system

1. XO – system. “O” is the symbol for no chromosome.
   • XX are females and XO are males.

   E.g. Sternoptyx diaphana, Oncorhynchus nerka,Coris julis

2. ZO-system. ZZ males, and ZO females.

E.g. Colisa lalius, C. fasciatus