You are correct in your assumption regarding D.J., but I think you are a bit over the top when trying to compare skin color of actors hired to portray characters in a television program to the exact probable genetic outcomes.
You could also complain that although D.J., Darlene, and Becky are siblings on the program, based on their skin tones and hair characteristics, they couldn't possibly be genetically related to each other, let alone be the genetic offspring of John Goodman and Rosanne Barr.
It's a television program, not a genetics class.
Why isn’t it a genetic class, it’s a LBGTIS class, an AAA class, a redneck class etc. Do it right.