Is there something I don't understand about sim genetics?

[Celia]

Is there something I don't understand about sim genetics?  I've been using Heleane's 3105 eyes as defaults by Corvidophile, plus some other non-default custom eyes.  Some of the children are turning up with what looks like the original Maxis brown eyes--upsetting for me, since I particularly dislike those.
Their parents don't have those eyes, in fact, in at least one family, both are cas made sims with custom eyes--yet the child has these ugly Maxis brown eyes.  The father has hazel eyes and the mother a dark brown.