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Zygote vs. Gamete
Someone who says this might think a zygote is just an egg cell with a sperm cell inside. Eggs aren’t morally valuable, and sperm aren’t morally valuable, so why should the combination be? In reality, a zygote is just one cell, not two cells. During fertilization, the sperm and egg break down and cease to exist! The sperm’s tail disintegrates. The sperm and egg’s nuclear membranes dissolve. The genetic material of the sperm and the genetic material of the egg fuse to create a totally new and unique set of genetic material. Neither sperm nor eggs contain a complete set of human chromosomes — they are both “haploid”. A “diploid” zygote, on the other hand, does have a full set of 46 human chromosomes.
The zygote is a brand new human organism. You can trace your history back to when you were a child, and further back to when you were an infant, and even further back to when you were a fetus, and all the way back to when you were a zygote. But the story stops there! It doesn’t even make sense to try to identify you before that point, because the egg and sperm you came from each was missing some of your DNA. You were once a zygote, but you were never an egg and you were never a sperm.