Final answer:
To determine DNA ancestry, scientists compare the DNA of modern-day individuals to DNA samples from ancestral populations that have been collected through various means.
Step-by-step explanation:
To determine DNA ancestry, scientists compare the DNA of modern-day individuals to DNA samples from ancestral populations that have been collected through various means. Although we don't have direct DNA from ancient civilizations like the Celts or Anglo-Saxons, we can estimate their genetic makeup by studying the DNA of modern-day people who share a common ancestry with these ancient populations. These comparisons allow us to make inferences about the genetic history and origins of different populations.