The differences between male and female sexes are anatomical and physiological. "Sex" tends to relate to biological differences. A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most organisms that create their offspring using sexual reproduction have two sexes.

Types 

XO type of sex determination 

• Other than autosomes, at least one X chromosome is present in all insects. 
•  Some sperms contain X chromosomes, while some do not. 
• Eggs fertilised by sperms having X chromosomes become females. So, females have two X chromosomes. 
• Eggs fertilised by sperms not having X chromosomes become males. So, males have only one X chromosome. 
•  Example of organisms with XO type of sex determination − Insects

● XY type of sex determination 

• Males have X chromosome and its counterpart Y chromosome, which is distinctly smaller. Hence, males are XY. 
• Females have a pair of X chromosomes. Hence, females are XX. 
• Example of organisms with XY type of sex determination − Humans and Drosophila

Male heterogamety − XO and XY types of sex determination are examples of male heterogamety. 

• In XO type, some gametes have X chromosomes, while some gametes are without X chromosomes. 
• In XY type, some gametes have X chromosomes, while some gametes have Y chromosomes.

Female heterogamety − ZW type of sex determination is an example of female heterogamety. 

• In ZW type, the female has one Z and one W chromosome, while the male has a pair of Z chromosomes.