Figure 1.

A directed acyclic graph (DAG) showing maternal and fetal genetic effects on offspring birth weight. Offspring birth weight can be influenced by both maternal and fetal genotypes, the former operating through the intrauterine environment and via fetal genotype. As fetal genotype is determined by maternal and paternal genotypes, to accurately estimate the association between maternal genotype and offspring birth weight we need to partition the overall genetic effect into maternal- and fetal-specific genetic components. This DAG shows the key pathways and assumptions of our analyses. We have not depicted the (unknown) paths through which fetal genotype would influence fetal growth and birth weight. Note, also that we are assuming that phenotypes relating to fetal genotype do not affect the intrauterine environment, whereas it is known that waste products form the fetus are transferred back across the placenta into the maternal circulation. If fetal genotype influences the production or transfer of waste products, this assumption will be violated.