If your employer doesn't like you and fires you, it isn't necessarily actionable discrimination. The specific categories which are protected are spelled out in particular laws, or statutes. Because gender, or sex, is one protected category, everyone is protected by at least one category.
Many people fall into more than one protected category.
None of these are protected categories. In other words, if the category of the discrimination isn't spelled out in a statute, the employee is not protected from that form of discrimination. Therefore, if the boss doesn't like you, but you don't know why, or the category isn't protected by law, he can fire you or not hire you for that reason. The essential consideration is why. What matters is the motive.