The historically purest Targaryens in terms of genetic bloodline purity were the earliest generations of the dynasty, starting immediately with Aegon I the Conqueror and his sister-wives Visenya and Rhaenys, who practiced strict sibling marriages straight out of ancient Valyria. As centuries passed and subsequent generations occasionally married outside their house into other Westerosi nobility, the overall genetic purity diluted significantly over time, leaving later descendants with much smaller percentages of unmixed Valyrian ancestry.