Final answer:
The XX-XO system of sex determination is found in some insects and other invertebrates. In this system, the heterogametic sex is the male (XO) and the homogametic sex is the female (XX).
Step-by-step explanation:
The XX-XO system of sex determination is a system found in some insects and other invertebrates. In this system, females have two copies of the X chromosome (XX), while males have only one X chromosome (XO). The presence of the Y chromosome, as in the XY system in mammals, triggers the development of male characteristics. Therefore, in the XX-XO system, the heterogametic sex is the male (XO) and the homogametic sex is the female (XX).