Fig. 1.

Human genetic inheritance and microbial inheritance. (A) In human genetic inheritance, information transmission is vertical, with 50% of each child’s genome content inherited from the father and 50% from the mother. Note that birth order is from left to right. (B) In microbial inheritance, transmission occurs largely in the first 3 y of life from proximal microbe-rich sources, notably family members, and in particular between near-birth siblings who provide a higher degree of access to their microbes. However, microbial inheritance is not limited to family members and can include other environmental sources, including family friends, nearby surfaces, and the local water supply.