Fig. 1

The key model that we argue against in this paper. Under this model, the genetic variations in both the human microbiome and genome are used to calculate the additive genetic variance component of narrow-sense heritability. In the strictest form of the model, microbial genetic variation can contribute to phenotype heritability even if it is horizontally transmitted (i.e., acquired from the environment). This is because the focus should be on the holobiont, the supraorganism of both microbiome and host, and its associated hologenome (the combined DNA of all constituents). The more lenient form of this model restricts the microbial genetic variation relevant to phenotype heritability to be the subset acquired through vertical transmission. The dotted arrow represents lower levels of vertical transmission of the microbiome relative to horizontal transmission