Dragon Ball Zero (Doujinshi) - Vol. 1 Ch. 1.5 - Bardock's Secret

[MangaDex]

 

[arcsalvo]

This comic is funny and all, but since Saiyans are a warrior race with high attrition and a not particularly high birth rate, it's said that there's only a few thousand on Planet Vegeta in total. Meaning that there's probably enough unintentional inbreeding to where similar hair and faces would become a thing due to a lot of shared DNA. Kind of like how in the past, people often didn't move more than 5 miles (or some other short distance) away from where they were born, and everyone was 4th cousins due to the lack of travel options (before modern transportation) leading to people inbreeding without knowing it due to not having a very high selection of potential mates.