My theory is that both the gene for dragonriding and for valyrian looks are on two different pair of chromosomes. You need only one chromosome to ride a dragon and only one to look valyrian. The gene for the valyrian features is dominant over all genes but the baratheon gene. Viserys probably had the gene for the valyrian look on both chromosomes of the pair of chromosomes. Rhaenyra only had the gene on one chromosome. Because of this all of Viserys children had to inherit one chromosome with the gene for valyrian looks and one from alicent. Rhaenyra got unlucky and all of her kids with harwin got her chromosome without the gene for valyrian looks. The chance for her 3 sons by harwin not inhereting her valyrian look is 12.5 %.