@DivinylsFan - The children of single parent sims who grew up to look as neither zero slider sims nor exact clones of their parent were not glitches, they were created as the game developers intended. The base game shipped with the 'genetics feature' already built into CAS it was not an updated add on feature. This feature had to be tested and assigned an in-game role beyond (if) the player would ever use it. Thus, the single parent infants are proof of its presence inside of an ongoing game. However, later in an update EA turned off giving infants to single parents so the genetic feature is no longer used in that way.
But a single parent only accounts for a portion of that sim's genetics. No one really knows how the game decides to fill in the missing genetic pieces of the non-existent parent. It is assumed the game pulls from a single source from the opposite gender but in truth it can pull from many sources as it chooses which may explain why those type of sims always grows up to develop their own unique look.
As far as Mortimer is concerned, which of his parents or neither of them he visually (not numerically) looks like is based on what you see in those sims. In my game, teen Mortimer (not all Mortimers) resembles his mother. This version of him is the one I have seen the most because he is always hanging out in my sim's home. In other games, Mortimer is a very chunky sim who looks like neither of his parents. And still, in other games his face is thin and his expression a bit forlorn closer to Agnes' than his parents.
Yes, the developers of the sims did in fact, play with some of them as they were developing them. Not in the way that 'we as players' play with them but rather as tester sims for all sort of programming and animation checks and a few of those sims actually made it into game with memories in tact. Some even have histories or backstories which were later removed or altered.