I'd like to see if we can revisit this discussion again. The participants in the highest voted meta question about this topic have all been absent for over a year, and I don't think the issue has gotten much better since October. Since making a commitment to spend more time helping this site, I've been reading through more questions and answers over the past few days. There are quite a number of answers with no references, references unrelated to their claims, and even references quoted but not cited.
Here's my opinion:
I agree strongly with @AliceD that references are necessary on a scientific stack. On SO you post the code, which is verifiable. On Biology, you include a reference.
The one exception, I would think, would be questions like this, where the asker already has the textbook and is asking for help understanding it. In this case, though, I think we should consider requiring the asker to reference their textbook, as the OP did in this case. (should I suggest this in an additional meta post?). Then the answer can refer to the same textbook. Even in this case, I personally like to include suggestions for additional reading, i.e., "you can read more about this here: source".
Textbook level information in general can be easily referenced with an authoritative textbook. E.g., this answer, is both correct and easily referenced. I would personally have chosen Thompson and Thompson Genetics in Medicine, Ch. 8, Complex Inheritance of Common Multifactorial Disorders. As experienced biologists, referencing textbook level information is not as hard as we think it is. If you can't remember where you learned something, you might not be remembering the thing you learned either. Not only do textbook references give the answers more credibility, they keep us honest and help reinforce the importance of references for new users.