Me: With an autosomal dominant trait, everyone who has the trait inherited it from a parent with that trait. Student: I have this condition which is listed as autosomal dominant. But neither of my parents have it. How does that work? Me:
Replies
This is why high school biology classes permanently nixed the "let's all find out our blood types" activity...
sep 9, 2025, 11:00 pm • 45 0 • view
We worked out the blood types as charts (no needles in class) and on one of them, we discovered that the only answer was “mom cheated”. My bio teacher apparently got a very stern lecture because the next day we were back to Mendel & his peas.
sep 10, 2025, 3:53 am • 4 0 • view
Same. When it came time for the blood typing activity in high school bio, my teacher was all "What a pity, the kits are all expired."
sep 10, 2025, 2:47 am • 12 0 • view
Always an awkward day...
sep 9, 2025, 11:29 pm • 1 0 • view
the dna stuff on ancestral and 23me have blown the log off a LOT of stuff. quite a few interesting articles about this among other things, incest is pretty easily determined
sep 10, 2025, 3:00 am • 10 0 • view
sigh—log shd be top
sep 10, 2025, 3:19 am • 5 0 • view
Shall you tell them, or shall I?
sep 10, 2025, 2:36 am • 0 0 • view
May I ask which trait in question? There are a lot of textbook examples of human Mendelian traits that are actually not. udel.edu/~mcdonald/my...
sep 9, 2025, 10:23 pm • 2 1 • view
I actually don’t know. The student had pulled up a webpage on their phone about it, and it wasn’t one I knew off the top of my head.
sep 9, 2025, 10:26 pm • 12 0 • view
De novo mutations are common in the rare disease literature. A lot of micro deletions lead to deleterious haploinsufficiency. I imagine this happens at benign loci as well. Could be an excellent case for when an exception to a 'rule' allows you to discuss even cooler science.
sep 10, 2025, 2:49 pm • 5 0 • view
Yay weird genetics. My daughters have an inherited missense mutation from their biodad, but the youngest also has a CAG repeat from me, and a de novo frameshift mutation on a different chromosome. Her phenotype makes Drs go WTF?
sep 11, 2025, 6:38 am • 2 0 • view
Sounds like she had a thorough genome scan. I find it amazing that we went from thinking that knowing the genome of homo sapiens was a moon shot to sequencing patients for diagnostics within my career. I thank the American tax payer for their investments in science that allowed this.
sep 11, 2025, 11:40 am • 1 0 • view
A research team did full exome sequencing. Its changing how doctors approach diagnosing gene events. Its been pretty amazing to be a part of.
sep 12, 2025, 5:14 am • 0 0 • view
Gonna be an interesting call home
sep 9, 2025, 10:16 pm • 3 0 • view
I went with “One possibility is that you are a mutant” and “Let’s explain the concept of trinucleotide repeat expansion diseases with incomplete penetrance”, rather than “How sure are you that those are your genetic parents?”
sep 9, 2025, 10:15 pm • 322 11 • view
I wish I could say it's unusual for people to still hide adoption from their adopted children but it sadly isn't. I feel bad for your student.
sep 11, 2025, 3:12 am • 2 0 • view
Ahh. I shall, then.
sep 10, 2025, 2:36 am • 0 0 • view
Note: I did not list the trait in question. The student pulled up information about it online.
sep 9, 2025, 10:20 pm • 174 1 • view
In this situations, do you calm the parents to give them a heads up in case the student asks questions? Or do you wait to see if they ask you how the topic came up?
sep 10, 2025, 3:40 am • 4 0 • view
I'm a college professor. The odds of me ever speaking to the parents are close to 0.
sep 10, 2025, 3:41 am • 34 0 • view
Fair enough
sep 10, 2025, 3:41 am • 0 0 • view
In fact, depending on the context of the conversation, that call might not even be legal.
sep 10, 2025, 2:38 pm • 11 0 • view
😬
sep 10, 2025, 1:37 am • 12 0 • view
This is also how some people learn they were donor-conceived. Sperm, egg, embryo, so many possibilities.
sep 11, 2025, 12:55 am • 0 0 • view
Indeed. I just didn't really feel that was the appropriate direction for me to go in class when someone asked. I assume competent adults are aware of the existence of adoption and gamete/embryo donation, so I addressed other biologically relevant possibilities.
sep 11, 2025, 12:58 am • 4 0 • view
I think your assumption is a little naive on the gamete front, given dd's experience, but that was 10 years ago so maybe things are better now. (dd has always known; I'm a single mom by choice.)
sep 11, 2025, 1:05 am • 0 0 • view
I guess I could amend that to "who are taking a biology-for-majors course in college", but certainly possible I'm being naive.
sep 11, 2025, 1:09 am • 2 0 • view
I included a disclaimer up front any time we used a student’s own info, that there were many reasons individual results might differ from theoretical ones, and while I might be *a* doctor, I wasn’t *their* doctor. If they had questions, they should discuss it with their doctor and/or parents.
sep 9, 2025, 11:18 pm • 34 1 • view
I used classroom examples today where I explicitly said “These aren’t actually 1 gene with 2 alleles, but for the purposes of this discussion let’s pretend” And then used not-medical stuff like whether you have hair between the knuckles on your fingers, or if your earlobes are detached.
sep 9, 2025, 11:22 pm • 29 1 • view
Ah yes. My mom has detached earlobes and my dad has attached. I am incredibly physically similar to my father, and my mom jokes that the only gene that I got from her that I actually express is earlobes.
sep 10, 2025, 12:12 am • 17 0 • view
no finger hair is an option?! today i learned…
sep 10, 2025, 4:09 pm • 0 0 • view
*grabs at ears* Ok. Ok. They are attached.
sep 10, 2025, 2:38 am • 1 0 • view
Teaching human genetics is always super fraught!
sep 9, 2025, 10:52 pm • 63 0 • view
My little sister is the only blonde/blue in our *entire* family tree on both sides. She's done genetics, so it wasn't the proverbial mailman, but we've got absolutely no idea where it came from.
sep 9, 2025, 11:28 pm • 18 0 • view
Yeah, anything color-based is *way* more complicated than it's made out to be at the K-12 levels.
sep 9, 2025, 11:31 pm • 47 1 • view
Recessive can hide.
sep 10, 2025, 2:37 am • 3 0 • view
And it seems plausible it was an easy mutation in the first place. We trace humanity to Africa, don't we?
sep 10, 2025, 8:53 am • 1 0 • view
this is why i rather do cat genetics, color-wise it's complicated but also cute cats
sep 10, 2025, 5:42 pm • 5 0 • view
sep 10, 2025, 12:07 am • 3 0 • view
Oh my partner had that happen! He teaches bio and told at least two kids their parents werent their parents through explaining genetic inheritance.
sep 10, 2025, 5:06 pm • 1 0 • view
Thing is, you can't make the determination based on just that one trait/allèle. It just raises the possibility, which is hard enough to broach. I really hope he didn't tell them their parents are not necessarily their biological parents on that basis.
sep 10, 2025, 7:58 pm • 2 0 • view
Oh, of course not. He talked about mutations and suggested they ask their parents about the specific trait and where it came from. Then went on to talk about how trait inheritance is Complicated. In both cases, it just happened to be that they were adopted and weren't told.
sep 10, 2025, 10:13 pm • 2 0 • view
And THIS is why we don’t have kids do their blood type family trees any more! 🤣
sep 9, 2025, 11:03 pm • 74 0 • view
At least those were better than telling them that eye color is a one locus 3 allele model.
sep 9, 2025, 11:16 pm • 45 1 • view
This is the best eye color model I know of, and even it gets it wrong sometimes. Also, you know, not suitable for high school bio class at. all. 🤣 hirisplex.erasmusmc.nl
sep 10, 2025, 2:29 am • 7 1 • view
Yet another good reason for my slides of "Reality is much more complicated than this, but for the purposes of this discussion, let's pretend..." Also, to be honest, this is why I prefer examples in *fictional beings*.
sep 10, 2025, 2:33 am • 10 0 • view
When I TAed a grad-level molecular evolution class, questions I wrote included genetic admixture between the elves of Mirkwood and Loth Lorien, whether star- or not-bellied Sneetches were the dominant phenotype, a genetic complementation test in unicorns...
sep 10, 2025, 2:33 am • 20 2 • view
Maaaaaan. The only fun I ever got to have was making my citation format example. Jones, H. W., Jr & Jones, H. W. Feasibility and Uses of Vessel Materials. J Magical Medicine 55, 7-13 (1939).
sep 10, 2025, 2:41 am • 3 0 • view
A lot of my colleagues prefer to use real examples of things. I generally worry that a real example could easily have extraneous stuff that complicates the concept we're trying to isolate. And may throw students who know about those complications.
sep 10, 2025, 2:45 am • 10 0 • view
I like real examples - I think it helps student to understand that stuff isn’t just a pointless homework exercise - but it’s *very* hard in genetics and neuro to give real examples that aren’t just a sharp ball of edge cases.
sep 10, 2025, 2:52 am • 10 1 • view
My mom once told me about a Barr body lab her old professor stopped doing in college. Too many Klinefelters revelations.
sep 10, 2025, 3:21 am • 9 2 • view
The number of people I know whose eyes are blue or blue-gray normally and green after hard crying is startling. (I don't know if there are people whose eyes stay blue after crying, and if so, if there's a non-crying way to tell the groups apart)
sep 10, 2025, 1:58 am • 1 0 • view
Mine are mostly brown that turn green after crying.
sep 10, 2025, 5:47 pm • 1 0 • view
Thanks! That means it's not just (some?) blue/blue-gray eyes that do that.
sep 10, 2025, 10:49 pm • 2 0 • view
We don't even know why hazel (multicolored eyes) are hazel. Having that trait throws a large spanner in even the 2 locus eye color model.
sep 10, 2025, 2:25 am • 5 0 • view
The standard (incorrect) way that's taught in the 1 locus 3 allele model is that hazel is a green/brown codominant situation. That isn't how it works in reality, of course.
sep 10, 2025, 2:27 am • 7 2 • view
Nope, found that out very quickly. It made no sense given the inheritance patterns in our family.
sep 10, 2025, 2:34 am • 1 0 • view
Dad had one kind of hazel (blue/amber central heterochromia), Mom another (brown/green scattered heterochromia). I went looking for why. All they could say was "It's complicated." 😄
sep 10, 2025, 2:25 am • 2 0 • view
However it worked (I have greenish-blue with a gold-brown center; spouse has blue), the offspring got the most amazingly excellent blue-gray eyes. I'm very pleased. ^_^
sep 10, 2025, 9:07 am • 3 0 • view
I always had at least one student a semester who felt absolutely betrayed when they found out this wasn’t true but their HS teacher had taught it anyway.
sep 9, 2025, 11:21 pm • 28 0 • view
I may have a bit of a soap box about that one because while my eye color shifts, it is most often reasonably described as "gray". Turns out the gray/blue difference seems to be mostly about the thickness of a particular type of fiber, and how much it scatters lights.
sep 9, 2025, 11:36 pm • 37 1 • view
Yeah, I have very distinct central heterochromia, which does weird things to my eye color. They're green with a brown center. They can look anywhere from green/gray to golden orred depending on lighting/shirt color. My parents have plain blue and plain brown. My brother has the same eyes I do.
sep 10, 2025, 12:17 am • 15 0 • view
Definitely had a "this is oversimplification but as An Example" for teaching high school bio. 🤣
sep 10, 2025, 12:22 am • 4 0 • view
My brother and father had/have vibrantly blue eyes. My mother had color-shifting eyes, but hers were most often green. Her sister has typically blue eyes. I used to look at ID photos, see whether mine were more blue or more green there, and then list as the other. Now I list as gray if possible.
sep 10, 2025, 12:34 am • 7 0 • view
Apparently, it's weird that my brother and I both have the same color eyes because it's usually a mutation! I just go with "hazel" but sometimes that's not an option! Then I'm stuck with green. 🫣 A friend's brother has absurdly bright blue eyes. They look almost fake! The variations are amazing.
sep 10, 2025, 12:49 am • 4 0 • view
My younger brother has those bright blue eyes too. We learned very young to say both our grandmas had blue eyes because my parents, sister and I all have dark brown.
sep 10, 2025, 9:36 pm • 1 0 • view
My dad gets Dune references from his.
sep 10, 2025, 12:55 am • 10 0 • view
My eyes are blue and green. I never know what to say when asked my eye color. It’s blue-green hazel. Not my fault if that’s not an option for the driver’s license.
sep 10, 2025, 1:02 am • 6 0 • view
If it's a mutation, it's heritable, and strongly so. My paternal grandmother had grey-blue eyes with amber central heterochromia, my dad did, and two of my sibs and I both do. The other two sibs had brown/green scattered heterochromia, same color as our mother's.
sep 10, 2025, 2:32 am • 1 0 • view
A perfect learning opportunity for “all models are wrong but some are useful”, maybe?
sep 9, 2025, 11:23 pm • 2 0 • view
"All models are wrong; some models will be used in this classroom". One of my friends wanted to title her dissertation "All models are wrong; some models are in this dissertation", which I say: go for it.
sep 9, 2025, 11:37 pm • 7 0 • view
I would have liked to read that dissertation - I bet it’s fascinating.
sep 9, 2025, 11:38 pm • 0 0 • view
The actual title is Computational Methods to Investigate Connectivity in Evolvable Systems by A. Ackles if you want to check ProQuest.
sep 9, 2025, 11:42 pm • 3 0 • view
My bio 101 prof discussed penetrance with a personal anecdote about comparing eye color in her immediate family and having an, "Oh no, my brother isn't really my brother!" moment before digging further and finding it was a case of incomplete penetrance.
sep 10, 2025, 12:19 am • 8 0 • view
In addition to that side eye, at a minimum you have allelic heterogeneity, locus heterogeneity, incomplete penetrance, de novo mutation
sep 10, 2025, 6:39 am • 1 0 • view
uh oh
sep 10, 2025, 2:16 am • 0 0 • view
“Well you see, when a bored housewife and a milk man love each other very much…”
sep 10, 2025, 1:26 am • 13 0 • view
hoooo boy it's always a fun time when I need to clarify the genetic relatedness of family members with an investigator, but thankfully that's a step removed
sep 10, 2025, 12:48 pm • 5 0 • view
Gentle gentle teaching helps in these circumstances.
sep 9, 2025, 11:43 pm • 4 0 • view
Sometimes it turns out to be a misdiagnosis! It was in mine and my sister's case.
sep 10, 2025, 1:31 am • 5 0 • view
I've heard multiple stories of students who found out they were adopted thanks to biology class 😬
sep 10, 2025, 1:13 am • 7 0 • view
What’s wrong with that? Biology is supposed to be useful. Unless you presume that there is something shameful about being adopted. Lots of women find out that they were, in fact, sexually assaulted during discussions in women’s studies class.
sep 10, 2025, 4:21 pm • 4 0 • view
I don't really think there's anything wrong with that. Maybe the adoptive parents would have preferred to admit it on different terms, I guess, but it's probably fine.
sep 10, 2025, 4:24 pm • 1 0 • view
Then they should have done it. Commercial DNA testing is coming for everyone’s reproductive secrets. It’s not the job of biology professors to uphold them.
sep 10, 2025, 7:04 pm • 2 0 • view
De novo mutation! Mosaicism! Anticipation! …😉
sep 10, 2025, 1:52 am • 4 0 • view
I would have to know what the trait is to say more, but a lot of supposedly simple traits (like eye color) can vary a great deal in their expression.
sep 10, 2025, 3:57 am • 1 0 • view
I'm aware.
sep 10, 2025, 4:00 am • 3 0 • view
I'm sure you are, which is why I find the whole side eye thing weird.
sep 10, 2025, 4:03 am • 1 0 • view
Prob because it is uncomfortable since one answer is the kid's parents arent their parents. Which any decent prof wouldnt want to bring up in class.
sep 10, 2025, 5:34 pm • 4 0 • view
Dominant/recessive genetics seems like the sort of topic literally everybody who teaches bio has a "student suddenly has an Awkward Question" suddenly raised story.
sep 9, 2025, 10:48 pm • 19 0 • view
Ope
sep 10, 2025, 2:54 am • 1 0 • view
I read recently that ~3% of humans have different genetic parents than expected. Did the mental math on my 180 person genetics class and decided that I probably shouldn't bring it up ;)
sep 10, 2025, 1:01 am • 46 2 • view
Back when I was in grad school lab was genotyping mosquito bloodmeals to identify who they fed on in SE Asian villages. Identities blinded but we knew familial relationships (child, spouse, etc). 3% is a good estimate (I think studies have shown that's pretty constant across populations & cultures)
sep 10, 2025, 12:03 pm • 5 1 • view
When we loop back into sexual selection during the Evolution unit, I will mention extra pair copulations. I plan to not use a human example. Birds work well for this! I similarly did not use a human example when talking about phenotypic selection, when the Brassicas are sitting right there.
sep 10, 2025, 2:47 am • 27 1 • view
In fact, @ngerlach.bsky.social, who's chimed in on the thread, is an actual expert on bird extra pair paternity :)
sep 10, 2025, 11:11 am • 2 0 • view
Obviously hard (not to mention wildly unethical) to get good numbers on this for humans, but I’ve seen estimates up to 10%!
sep 10, 2025, 3:04 am • 2 1 • view
I likewise remember reading that relatives of the mother often comment about how much a newborn resembles some relative of the father, while medical personnel agree most newborns look pretty similar.
sep 10, 2025, 3:16 am • 6 1 • view
Coworker: I met your dad at your defense. You look just like him. Me: I do resemble him, but I actually look substantially more like my mother. Coworker: No, you really look like him. Me: Let me show you a picture. Coworker: Oh my God, you're your mother with glasses and a square jaw.
sep 10, 2025, 3:24 am • 13 1 • view
they… they look like each other
sep 10, 2025, 3:25 am • 5 0 • view
the technical term is "assortative mating"
sep 10, 2025, 3:33 am • 2 0 • view
What's that?
sep 10, 2025, 5:42 pm • 0 0 • view
It's when individuals within a species preferentially mate with those who are like them in some way. So for example, assortative mating by size would mean that the large individuals mated with other large individuals, and the smallest with the smallest, etc.
sep 10, 2025, 6:10 pm • 4 0 • view
There’s an amazing opposite example of this in white-throated sparrows who practice “perfect disassortative mating” and only mate with the opposite color morph.
sep 10, 2025, 7:17 pm • 14 1 • view
In this case, assortative mating on educational attainment and facial features. For my parents: not on size. Mom was a bit under 5'. Dad was 6'2 (probably ~6'1 now, in his mid 70s). I am a towering giant on my mother's side, and the runt of my generation on my dad's.
sep 10, 2025, 7:26 pm • 5 0 • view
Oh I see, thanks!
sep 10, 2025, 10:14 pm • 0 0 • view
I like this photo as an example of assortative mating (and its consequences) within each of the bridal families, and dissortative pairing between the bride and groom.
sep 10, 2025, 7:36 pm • 4 0 • view
They really do. Interestingly, more than my brother and I resembled each other.
sep 10, 2025, 3:34 am • 3 0 • view
One of the (many) reasons I use cat fur/trait genetics to illustrate those principles in my #BI111 class (with the bonus I get to talk about of the ear phenotypes of Benjamin Buttons (L) and Olivia Benson (R))!
sep 10, 2025, 11:56 am • 5 1 • view
Agreed. The 3% comes from a survey of people who did direct-to-consumer genetic testing, who might not be representative of the general pop (cited here: www.newyorker.com/magazine/202...)
sep 10, 2025, 11:13 am • 1 0 • view
Well, back in the 50s some anthropologist studied upper middle class WASPy families in a town around Pittsburgh somewhere. They found that in pretty much every family the first kid was definitely the husband's, the second was probably the husband's, the third was 50/50, and any other was a toss up.
sep 10, 2025, 5:44 pm • 0 0 • view
He couldnt publish til the 2000s because it was so controversial. But the home DNA is proving him right lol.
sep 10, 2025, 5:44 pm • 0 0 • view
Here's a meta-analysis from 2006: 1.9% EPP in samples where the men had high paternity confidence and ~30% EPP in samples where the men had low paternity confidence (e.g. had requested paternity analysis be done). So 3% seems reasonable for the general population.
sep 10, 2025, 6:08 pm • 1 0 • view
Lolol. My husband used to teach an anthropological genetics course that spent a couple classes on what happens when DNA tests for genealogy or fun turn into unwelcome surprises.
sep 9, 2025, 11:19 pm • 21 0 • view
Time to have the talk
sep 9, 2025, 11:01 pm • 0 0 • view
Another fun possibility: I have a condition that's autosomal dominant and "neither of my parents had it"... until sufficient questioning determined the exact path it took through the family lineage, because everybody had brushed it off as a family quirk.
sep 9, 2025, 11:01 pm • 35 0 • view
That’s part of why I talked about incomplete penetrance. Sometimes people with a more mild presentation of a trait never realize they have it until someone else in the family has a more extensive version.
sep 9, 2025, 11:03 pm • 40 0 • view
Also, my example trait in the slides was Huntington, which is an autosomal dominant trinucleotide expansion disease with incomplete penetrance and very rarely visible before 40.
sep 9, 2025, 11:04 pm • 27 0 • view
There are a lot of disorders with highly variable penetrance in human genetics...
sep 11, 2025, 12:42 am • 2 0 • view
I graduated from MSU's Clinical Laboratory Science degree program in 1995. Previous classes would set up booths in the mall (I think it was the mall) to do blood typing. They had to stop because of similar issues.
sep 10, 2025, 11:20 am • 2 0 • view
Traits can be environmentally dependent (as I know you know!). So if neither of their parents has said condition, parent could still have dominant allele. They'd need to be genotyped.
sep 9, 2025, 11:02 pm • 14 0 • view
Yep. My classroom example of that today: the difference in adult mean height in North Korea vs South Korea. The variation in most wealthy countries is 80-90% genetic. But the difference between North and South Korea is nearly entirely environmental.
sep 9, 2025, 11:07 pm • 42 0 • view
Nice example. Have you got links for the nk vs sk data?
sep 10, 2025, 2:14 am • 2 0 • view
This is from 2009, but: dx.doi.org/10.1017/S002...
sep 10, 2025, 2:17 am • 15 0 • view