Limitations Of Consumer DNA Tests

We humans inherit two copies of nearly every gene from our parents.

Consumer DNA tests show us the pairs of alleles at specific positions on each chromosome. This, of course, isn’t the full genome. The consumer DNA tests sample a small set of specific markers to keep the costs affordable.

Here’s an example from my raw DNA results from Ancestry. The results show the allele pairs at four positions from the first chromosome.

  • A            G
  • T            C
  • G            G
  • C            T           

Unfortunately, this level of testing can’t assign a specific nucleotide to a specific chromosome. Consider the first AG allele pairing. I don’t know if A came from my father and G from my mother or vice versa.

The reality may have the alleles paired differently (e.g. reversed):

  • G            A
  • C            T
  • G            G
  • T            C

But notice one thing in the example where I simply reversed the order. The same alleles exist on the same chromosome e.g. ATGC is either on one of the chromosome pairs of the sequence is on the other.

That would be very useful information, but we don’t even know this with consumer DNA testing. Because the reality could be this:

  • G            A
  • T            C
  • G            G
  • C            T

If any of these three scenarios could be true, then we don’t know the real sequencing. Is there any solution to this problem? Yes, the answer is phasing.

Phasing uses intensive statistics to extrapolate which alleles belong on which chromosome.