I’m a biology teacher at a high school. Back when I was a much newer teacher, we had reached the point of learning about genetics and Punnett squares, which are used to show how dominant and recessive genes will be inherited from parents to children. Usually, the second most common example used for these — next to Mendel’s original example with peas — is eye color. However, I didn’t like this because it’s not accurate; eye color is actually controlled by sixteen different genes and is more complex than simple Punnett squares can handle.
Thus, I decided to use blood type as an example for our Punnett squares. It’s a slightly more complicated example, due to A and B types being codominant, but at least it doesn’t require lying to students.
Then, one day, a girl came up to me before the start of class.
Student: “Mr. [My Name], I think I’m doing the squares wrong.”
Me: “What’s the problem?”
Student: “My dad is AB and my mom is A, but I’m O. I can’t make the squares work.”
Me: “Oh, yes, that wouldn’t usually work. Are you certain you have everyone’s blood types right?”
Student: “Yeah, I asked them last night.”
Me: “Oh, I see.”
Student: “An AB parent can have an O kid, right?”
There was a hint of anxiety behind this question, as if she was pleading with me to tell her they could.
The answer — as I found out only after I was put on the spot with this question and Googled it — was that it is possible but exceptionally rare for this to happen. Still, even if I didn’t know that for certain yet, I knew there were usually exceptions to most genetic rules of thumb, so I hedged a little.
Me: “Usually not, but genetics are strange; there are always mutations or unusual recombination happening, so most of the stuff we teach in genetics is how things usually work, not a promise it will always be that way. I wouldn’t be surprised if there are times when AB parents have O children.”
Student: “But how often does that happen?”
Me: “I’m not really certain.”
Student: “But is it common?”
Me: “I don’t think it’s common, but it could happen.”
The student was quiet for a bit while thinking, looking increasingly worried. Eventually, she spoke up again.
Student: “What about eye color? Brown is dominant, right?”
The girl had brown eyes.
Me: “Eye color is more complicated since it’s controlled by many genes; Punnett squares don’t really work with them.”
Student: “So, brown really isn’t dominant?”
Me: “It kind of is. If you have one parent with blue eyes and one with brown, you’re more likely to end up with brown eyes, but it doesn’t always happen that way.”
Student: “But parents with blue eyes can have a brown-eyed daughter?”
Me: “Yes, definitely!”
Student: “Okay, good. Is that common?”
Me: “Well, it’s not exactly common, but it definitely can and does happen. I’m afraid I don’t know the exact odds without looking it up.”
Student: “Oh, okay. Thank you.”
She still looked worried and a bit dejected as she wandered back to her seat. I was not at all happy with how the conversation had gone, but I couldn’t think of anything else I could say or do, other than lying to her, to make things better, so I had to just leave things be and get on with starting class.
The next Monday, I saw the student again. She was looking very upset while looking at me as if she couldn’t decide whether to approach me or not.
Me: “[Student], are you okay?”
Student: “No! You were right!”
Me: “Right about what?”
She looked like she was considering answering, but she glanced around the classroom that was starting to fill with students, some of whom were clearly interested in our discussion in front of the class.
Student: “I don’t want to talk about it here.”
Of course, I respected that, but I found her to talk in private later. It turns out the man who raised her was not her biological father after all. He had fertility issues, and in the end, his brother ended up donating sperm for them to get pregnant, but they had never told their daughter. The blood types not adding up was the thing that made my student start thinking, but ultimately, she had come up with lots of other things, like eye color, that didn’t quite add up, and so she had decided to confront her parents during the weekend. They admitted the truth when she confronted them.
She was very upset at having this hidden from her, especially since, apparently, her biological father had moved to another country shortly after she was born, so she barely knew anything about him. I did my best to reassure her and remind her that anyone who cared for and loved her all her life was her real father, regardless of genetics, but she was less concerned about that and more about feeling like she had been lied to her entire life.
She would eventually come to accept the news, but not until she had time to deal with her feelings and emotions. As for me, I decided that I wasn’t going to be teaching any blood type-related Punnett squares after that year. I’ll still sometimes use the old standard of eye color but only after stressing that it’s an oversimplification. I figure, that way, if I ever have a brown-eyed student asking why their parents are both blue-eyed again, I can at least honestly say that is a real possibility and that I had already warned them we were oversimplifying eye color. I’d prefer not to be the cause of any more children getting unpleasant realizations.
