The reproductive cells involved in the process of reproduction are called gametes. One gamete does not contain the entire genetic material to form the diploid, hence, to form a zygote, which is the first stage in the development of a genetically unique organism, male and female gametes need to combine together.
Hence, the common trait of the sexual reproduction in organisms is the union of the haploid gametes to form a diploid zygote. In the advanced forms of heterogamy, one type of gamete is called sperm (animals) or pollen (plants) and the other type is called egg (animals) or egg cell (plants). The sperm and the egg have morphological differences.