How does fertilization combine two gametes to form a zygote?

Fertilization is the process where one haploid sperm and one haploid egg fuse to form a diploid zygote. First, the sperm undergoes capacitation and the acrosome reaction to penetrate the egg’s protective layers. 

The sperm and egg plasma membranes fuse and the sperm nucleus enters the egg. The haploid male and female pronuclei then merge (karyogamy) to form a single diploid nucleus containing a full set of chromosomes. 

At this point the cell is called the zygote (2 n). This event restores the species-specific chromosome number (in humans, 23 from mother + 23 from father = 46 total). Thus fertilization integrates one genome from each parent into one cell, initiating embryo development.