Zygote

A zygote is the first cell of a new organism that reproduces sexually. It forms at fertilization, when a haploid sperm and a haploid egg combine, so it is diploid: in humans it has 46 chromosomes, 23 from each parent. Inside the egg, the nuclei from the two gametes, called pronuclei, move toward each other, their envelopes break down and their chromosomes come together, completing fertilization. Its cytoplasm, including the mitochondria, comes almost entirely from the egg.

In humans, fertilization usually happens in the uterine tube. Within about a day the zygote starts cleavage, a series of rapid mitotic divisions that produce ever smaller cells, called blastomeres, without increasing the overall size. By about day 3 or 4 it is a solid ball called a morula, and it then becomes a fluid-filled blastocyst that implants in the uterus around day 6. The zygote is totipotent, able to give rise to every cell of the body and to the placenta.

Identical twins arise when the early embryo from one zygote splits in two, while fraternal twins come from two separate zygotes. Errors in egg or sperm formation can give a zygote too many or too few chromosomes, as in trisomy 21.