an organism with different cells emerged from the same zygote is mosaic and an organism composed of different populations of genetically distinct cells originated from different zygotes is chimera
Pl. refer to some standard Genetics Books in animal Science to understand. Simply understood, a strange occurence or bizarre appearance deviating from the normal
A mosaic contains cells of different genotypes that all originated from the same zygote (e.g. X-inactivation in female mammals). A chimera contains cells of different genotypes that originated from different zygotes (e.g. Freemartin heifers).
I agree with Urfer's opinion and again stress on the appearance of an unusual genotype deviating from the normal and a chance occurence which is unexpected. Survival rate is also less.