They would want someone of good genes to marry into their family to have those desirable traits to be inherited by the children and their children.
If they had that type of mentality in mind or had some form of other influence in their way of choosing a partner for themselves or their families, then they would pick out someone healthy with no existing problems in their family line.
There would be other ways of choosing, including the overall beauty of that person, their intellect, and many more factors that could be used to judge whether they were worthy of marrying into the family.
If they were believed to possess good genes or a good lineage, then they would be a good choice for such selective breeding.
It was the case for various families gathered around the capital that still did those kinds of things.
