Answer:
My best guess is "The hemoglobin gene contains a mutation".
Step-by-step explanation:
Sorry, I'm not an expert so bear with me!
One more common hemoglobin-related mutation is sickle-cell anemia. It occurs when the amino acid in hemoglobin is substituted from glutamate to valine. It is considered a mutation and is a direct swap from one specific amino acid (glutamate) to a different amino acid (valine) in the same position in the hemoglobin. So, considering that our example (sickle-cell anemia) fits the criteria of swapping one amino acid for another in the hemoglobin, is a mutation, and is passed down genetically (meaning that the hemoglobin gene would need to carry the mutation) it does look a bit like it fits your parameters!
Thus, as I understand it the hemoglobin gene carrying a mutation would make sense as the most likely cause of the amino acid variation.
Again, this is based on 10 minutes of googling. I hope this helps, but apologies if I'm incorrect!! Good luck :)