How does the placenta keep out the mother's blood but allow inherited characteristics through?
The placenta functions like a garden fence: it keeps the major components of the mother's blood from crossing to the fetus, but allows the small chromosomes and genes that carry inherited characteristics to pass through. Consequently, the child's blood system comes from the father, not from the mother's blood.
Les teaches this using the analogy of fencing a garden. Netting around a garden keeps out rabbits, dogs, and larger animals, but insects and smaller mice still get through. He says the placenta in the mother's womb works exactly that way: the major components of the mother's blood never go through the placenta to the fetus, but some small chromosomes and genes carrying inherited characteristics do go through. He clarifies that when Mary was the physical fleshly mother of the Lord Jesus, none of Mary's blood, which was just as corrupt as any other woman's blood, coursed through the baby Jesus. Instead, His blood system came from His Father, just like any other child's blood system comes from the father.