In Denmark, a husband and wife who had been unsuccessfully trying to have a baby went to a fertility clinic. Sperm and eggs were collected from father and mother, and
combined in a petri dish, creating several "test-tube babies". These babies were implanted in the
mother and 9 months later she delivered twins, one with lightly pigmented skin and one with
darkly pigmented skin. Because this seemed strange, a DNA test was conducted and it was
found that both children were related to the mother, but only the light skinned child was related to
the father. How can this be explained? (Hint, there was no infidelity)