Firstly, does the DNA line really affect the character's genetic (game) traits? From what I understand, the DNA line affects the hair/eyes color and facial features (so characters tend to look like their parents) and the properties line determines their haircut, expression, etc, with te genetic traits passing on to the children or not being a simple probability calculation, but I may be wrong on this.
Well, if my experiences with the eugenics system in the game has proven me anything, is that it's so random you might as well just stop trying to manipulate it.
Once I got a genius heir so I decided, to maximise the benefit, to get a genius wife for him. Took me a while but I figured the best pedigree in my game was some daughter of a mayor in finland (I was in based in Italy). It took me a while but eventually I got my heir married to her. 2 genius right, what can go wrong? Very much, 3 imbecile daughters and, thought this is just bad luck, it took 7 children to finally get a male heir. Not one genius or even any positive genetic trait showed up in their offspring.
Now in my latest campaign, just for quicks, I edit the save file before the first unpausing and gave both my character and his wife all the good genetic traits avaliable (genius, strong, fair, you name it) to see what would happen. They got 13 children, all of them got around 2 good genetic traits, fair being the most common, then strong, only one genius, and he went to 28 martial (sadly he died in his 20s in battle

). I made an effort to only marry women with good genetics but being on the 3rd generation, my king has no good genetic traits anymore and looking at the rest of the family, only a fair here and there remains, most are neutral. Also I should mention the last daughter of the original rulers (the ones I edited in their traits) actually managed to defy her impossibly good pedigree and got an ugly trait. She died in her infacy tough.