Let’s assume that Targaryen silver hair is a recessive trait. And we know that they like to marry brothers/sisters. I think the real reason that they like to intermarry is to keep their bloodline pure. There is a theory that their ability to control/hatch dragons is related to their genes, and it’s probably a recessive gene, too.
Someone did a fascinating analysis of the trait as X linked.
One X chromosome = dragon rider
Two X chromosomes = dragon hatcher
And someone figured this out, hence the murders / disappearances and forcing of specific princesses to become septas (nuns). Then the men of their lineage didn’t matter because they couldn’t father sons to ride dragons or daughters to hatch them.
Under this theory, Rhaegar found out about skin changing / warging in the Stark lineage and suspected it was similar to whatever magical gene let you ride/hatch dragons (telepathy?), so that’s why he wanted Lyanna. To hatch children that could bring back the dragon hatching ...