Fig. 2

There is currently no clear rationale for why monozygotic twins would acquire more similar strains through vertical transmission compared to dizygotic twins in early life. This would be required for the vertical transmission of microbes to contribute to the problem of missing heritability, because traditional heritability estimates have been based predominately on comparing differences in phenotype concordance between monozygotic and dizygotic twins. A contrasting example is epigenetic signals (indicated by grey dots on DNA), which have also been suggested to be a partial explanation for the issue of missing heritability. Although this point remains highly controversial, the rationale is nonetheless much clearer because any such signals would be transmitted through the zygote genomes to each twin