Sequencing an individual’s genome, while not yet exactly trivial, has by now passed into the realm of the feasible. But the results do not yet delineate which copy of a gene came from which parent. This is important not just so you can decide who to blame for your annoying laugh or weird double-jointed elbows; some genes are imprinted, meaning that they are only expressed from either the maternal or paternal chromosome. The gene encoding insulin-like growth factor 2 is the classic example—only the paternal gene is used. So if you have a normal allele from your mom but a mutated allele from your dad, you’re kind of screwed.

The “next generation sequencing” methods work by first cutting the DNA into small fragments, amplifying and sequencing those, and then piecing the results back together. These fragments are a mixture of maternal and paternal chromosomes, and there is no way to know which fragments came from which.