Igf2 is a growth promoter. Igf2r is a growth inhibitor. One is inherited from each parent and their effects balance each other out.
If the father copy of Igf2 is deleted then growth becomes stunted.
If the mother copy of Igf2r is deleted, growth is abnormal.
Need to have both as their interest differ.
If both are deleted embryo is normal as neither gene has other functions.