Is there a specific field in dog details that you're having an issue with? It's true, there is not a blank selection in a drop-down menu. All you have to do is highlight the entry you had chosen and click delete on your keyboard, then choose a different entry if you like in the drop-down menu.
Could not highlight and delete. Specifically, I had selected something under breed for a dog in the pedigree of my dog by mistake as breed was not specified in the pedigree. I have now, however, added an "unknown" option under breed so if I select the wrong one, I can switch to unknown.