Lou Charlton of DNAPrint Genomics explains that, genetically speaking, sex can be messy.
Charlton's company can analyze not only DNA from maternal or paternal lineages, which aren't mixed in offspring, but also the rest of a person's DNA, which is combined randomly from the mother and father every generation.
Charlton compares the process to trying to re-order the peas and carrots in a bowl of vegetable soup that's been tossed onto the kitchen floor. "It's not easy," she says. "Each generation, when the man and the woman mate, the bowl gets dumped on the floor again." [Link]