The words "biology" and "basic" don't go together. I'll explain here:
Let me tell you why people have genders in the simplest terms (not including twins, disorders, or gender stuff). In the beginning, the mother holds microscopic proteins. The sperm that the father in a sexual relationship gives the mother contains even more of these microscopic proteins. These microscopic molecules are called DNA (short for deoxyribonucleic acid), and the first sperm to reach the mother's egg has its DNA given to the egg. DNA is organized into sections of coding called "chromosomes". Regularly, humans have 23 pairs of chromosomes, or 46 chromosomes in general. One chromosome holds an X code that the mother gives, while the father can give either an X or a Y chromosome to pair with it. If an X is given, the offspring will be female (XX). If a Y is given, the offspring will me male (XY).
Now tell all that to a 5 year old. If that 5 year old gets it, it's basic.