Valyrian traits are dominant, Stop pretending they aren't

Maybe the issue is that we're trying to apply the rules of genetics from our world to what happens in ASOIAF.

In our world, dark hair and dark eyes are generally dominant traits. Yet Catelyn had auburn hair and blue eyes, and all but one of her children with Eddard inherited her appearance.

Now let's look at what happens when a Targaryen marries outside a Valyrian family:

• Daella Targaryen and Rodrik Arryn: We aren't certain what their children looked like, but the AWOIAF wiki depicts Aemma as blonde.
• Viserys I and Alicent Hightower: 100% of their children had the classic Valyrian look.
• Daeron II and Myriah Martell: Roughly 50% of their children displayed Valyrian features. That's exactly what you'd expect if Myriah's Dornish traits were recessive while Daeron carried dominant Valyrian traits, along with recessive non-Valyrian alleles (perhaps inherited through the Arryn line).

Exceptions:

• Rhaenyra Targaryen and Harwin Strong: I consider this simply bad luck. If you flip a coin three times, you can still get the same result every time, even if it's relatively unlikely.
• Aemon Targaryen and Jocelyn Baratheon / Aegon IV and Barba Bracken: These cases almost suggest codominance. Usually, both hair and eye color are inherited predominantly from one parent in ASOIAF, so offspring inheriting one trait from each parent is unusual and doesn't fit the apparent pattern very well.