The fertilized ovum (egg) before it begins to divide. The spermatozoon (SPERM cell) and the ovum are each haploid cells (gametes); they contain half the complement of chromosomes necessary to create an organism. When two gametes join they form a single diploid cell that contains the full complement of chromosomes. The zygote then divides as a haploid cell, becoming a blastocyst and eventually forming an EMBRYO.
For further discussion of zygotes within the context of the structures and functions of genetics, please see the overview section “Genetics and Molecular Medicine.”