The race is usually connected to how a person looks physically. For example, you will know what a person’s race is based on the shape of her eyes or the color of her skin. You always cannot tell what a person’s ethnicity is just by looking at the person. Ethnicity is based on a person’s nationality or a person’s culture. It is true that you can only have one race, even if it is considered to be “mixed.”
People can have different ethnicities because people’s upbringing will always be different from each other. Even those who live in the same household may sometimes have different ethnicities based on their experiences and the things that they were exposed to.