Simplify the following text for a Grade 10 native English-speaking person.
Original: Zygotes are usually produced by a fertilization event between two haploid cellsâ '' an ovum from a female and a sperm cell from a maleâ '' which combine to form the single diploid cell .
Simplified: Zygotes are produced by fertilization between two haploid cells , the ovum and the sperm cells , which make a diploid cell .