The article doesn&#x27;t answer the question in any satisfying way. A layman would probably believe that the environment in the uterus is the same for both twins, too. The &quot;satisfying&quot; answer is still quite boring: as with everything else, there are stochastic processes in play during embryonic development. For one, cytoplasm is divided asymmetrically, so the two embryos don&#x27;t begin their separate journeys with a perfectly equal share of their progenitor&#x27;s resources.<p>Also, this is a rather silly sentence:<p><i>Although very similar, the fingerprints of identical twins are nevertheless unique because they are not completely determined by genetics: they are an example of phenotype, which refers to the physical characteristics of an individual that are determined by the interaction of genes and the environment.</i><p>By this reasoning, the only thing <i>completely</i> determined by genetics is genes, since every manifestation of genotype could be called a phenotype.