This was the intended use from the beginning. Everyone know dna from both parents is used . This means the signal decreases by half each generation. Anything under 7 percent is unreliable. Basically, you get as much information as talking To your relatives and doing some basic genealogy.
They also look at mitochondrial DNA, passed on from mother to child. This actually stays identifiable through thousands of generations, passed down through the female line. (Males cannot pass this on.)