Since all of a child's DNA comes from the parents, it's easy to identify which parent contributed what. They take the child's DNA, cut it up with an enzyme, then "print" it on a gel by running electricity through it. The longer the segment, the slower it takes to gravel through the gel. They cut each of the parent's DNA with the same enzyme and run it through a gel as well. They then compare the results. All of the child's segments should match up with both parents. If doing just one parent, then roughly half of the segments should match up with the parent.
at 12:37 AM on Jun. 6, 2011