A healthy human zygote has a total of 46 chromosomes: 23 from the egg and 23 from the sperm. While it is relatively rare, there are cases where the zygote will have an extra chromosome, which can cause a variety of birth defects like Down syndrome.
Chromosomes contain all the genetic material of an organism. When a zygote is formed, it contains chromosomes, and therefore genetic material, from both the father and mother. Because a zygote has two distinct sets of chromosomes, it is considered a diploid cell. The sperm and unfertilized egg are individually considered haploid cells because they have one set of chromosomes.