Human Reproduction specifically when an egg splits and you end up with identical twins

o If a disease, what is its cause, how does it manifests, prevalence, risk factors, prevention, treatment etc.
o If a type of organism, what is its classification, characteristics, habitat, its role in the ecosystem, any human use (or abuse), importance etc.
o If a substance, what is its chemical nature, where is it obtained from (or how is it made), how does it act, its relevance, use etc.