Britannica Sources used in this answer

How does fertilization create a zygote with genes from two parents?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

embryos of different animals — Animal development The embryos of many animals appear similar to one another in the earliest stages of development and progress into their specialized forms in later stages.

Fertilization creates a zygote with genes from two parents by uniting two haploid gametes—the sperm and the egg—each of which carries only one set of chromosomes.123

During the process, a single sperm penetrates the egg, and its head transforms into a male pronucleus, while the egg nucleus matures into a female pronucleus. These two pronuclei meet within the egg cytoplasm and lose their nuclear membranes, allowing their respective sets of chromosomes to reassociate.1

This union restores the full diploid number of chromosomes, providing the basis for biparental inheritance. Because each parent contributes a haploid set, the resulting zygote is genetically unique, containing a complete set of instructions for the development of a new individual. This restoration of the diploid state is a fundamental requirement for sexual reproduction in animals.143