The monozygotic twins develop from the same fertilised egg. Hence the genetic composition or the make of both the twins are same. The differences in the height, weight, behaviour, and complexion of the twins exist not because of the underlying genetics but because of the environmental factors affecting the genes' activity.
The regulatory markers (chemical markers like the presence of methyl or acetyl group on the nucleotides) present on the identical genes present in both the twins differ. This is the epigenome of the twins. Epigenome is influenced by several external factors and affects the amount of gene products based on the regulatory markers over the gene.
