Monozygotic twinning is when a single sperm fertilizes a single egg and produces a single zygote, but in the earliest stages of development that embryo splits into two organisms.
|Image from Gilbert & Barresi, Developmental Biology, 11th Edition|
We don’t actually know why this happens. MZ twinning is more common in in vitro fertilization (IVF) than in naturally occurring pregnancies, and there is some interesting research trying to explore why that might be, but so far we haven’t got any clear answers.
|Fred & George prove mysterious once again.|
But we have analogous situations where a single organism can ultimately produce another, and that doesn’t undermine the fact that the original organism was and is a single organism.
For example, some people think of MZ twinning as akin to a type of asexual reproduction. Flatworms can eventually generate and split from new flatworms. That doesn’t mean the original flatworm wasn’t an individual organism.
|The parent flatworm on the far left is an individual organism.|
|Donor 1 (top left blue cow) is an individual organism.|
This is a fascinating and impressive process, but it doesn’t change the fact that Donor 1 is an individual organism. We wouldn’t look at the donor cow and say “Is she an organism? Is she just one organism? If bizarre or mysterious processes happen to some of her cells, it’s possible she could generate a new organism, so how do we know how many organisms she is?” No, we know she is a single cow.
By analogy, the zygote may, for reasons mysterious, generate an additional organism; that doesn’t change the fact that the zygote is a single organism, and that a human zygote is the first developmental stage of a human organism.