When two mice both carry an unmarked copy of a gene, their offspring should carry an unmarked copy too.
One copy from each parent, combined in predictable ways. Simple math, reliable outcome.
Researchers recently put that assumption to the test across three generations of mice, tracking the chemical tags that switch genes on or off.
The results didn’t add up the way the textbook says they should.
Beyond Mendel’s pea plants
The team behind the work wanted to know how often the standard rules of inheritance actually hold up.
They focused on something Mendel could not see in his pea plants: DNA methylation.
The tags work as a chemical switch system, attaching to DNA and turning specific genes on or off without changing the genetic code itself.
They sit on either copy of a gene, and they pass from parent to child.
Dr. Andrew Feinberg, Bloomberg Distinguished Professor at the Johns Hopkins University School of Medicine, co-led the research with colleagues at Texas A&M University.
Tracking inheritance in real time
To follow how the tags travel, the researchers bred three generations of laboratory mice – 26 in the founding group, 34 in the next, and 19 in the third.
The experts sampled liver and muscle tissue from each animal and read both the genetic sequence and the methylation pattern on the same DNA molecules.
Long-read sequencing made the dual read possible.
It captures far longer stretches of DNA than older methods could, letting the team see methylation tags sitting far from the genes they regulate.
Most of what the team saw lined up with Mendel. About 93% of the methylation patterns followed his classic rules in some form. The remaining 7% did not.
That came out to roughly 522 sites where the tags broke from the script.
In addition, 54 of those sites showed something stranger: methylation tags in offspring that were nowhere to be found in either parent.
A pairing between two mice with no methylation on a particular spot in the genome should produce offspring with no methylation on that spot.
The team kept finding offspring with methylation on both copies.
Evidence of natural paramutation
The strangest result involved a phenomenon called paramutation. The methylation on one copy of a gene somehow gets copied onto the other copy.
Both copies are tagged, even when only one parent contributed the tag.
Researchers had documented this in plants, flies, and genetically engineered mice – but never in mice with no genetic tampering.
A possible link to fertility
The paramutation showed up in a gene that helps drive normal sperm development in mice and humans.
Earlier work has tied reduced function of the human version to infertility.
That puts the new finding in fertility-research territory – not as a definitive link, but as a pattern worth tracking.
Genomic imprinting is one well-known way that inheritance breaks from Mendel’s rules. A tag from the mother silences her copy of a gene, or one from the father silences his.
The silencing depends on which parent contributed the copy, not on which version of a gene is stronger or weaker.
Feinberg’s team identified five additional genes that behave this way, expanding what had been a thin catalog in the mouse genome.
Environmental pressure and epigenetics
Methylation tags can come from outside the genome. Diet, stress, and environmental exposure have all been linked to changes in methylation patterns in earlier research.
Some of those changes can pass down to offspring. That makes the new findings unusual.
Epigenetic inheritance that changes in response to the environment could spread through a population faster than waiting for a random change in the DNA sequence itself.
“Non-Mendelian patterns of inheriting epigenetics could be a faster way to acquire diverse or new traits than alterations in the genomic sequence itself, especially in response to environmental pressures,” said Feinberg.
Expanding the rules of inheritance
Until this study, paramutation in mammals had been a laboratory curiosity. It was produced only through direct genetic engineering.
These mice showed it happening on its own, in a genome no one had altered.
Feinberg and his colleagues now plan to apply the same approach to human genomic data.
The team will look for similar patterns in families to confirm whether this kind of inheritance occurs in people the same way it was just found in mice.
Clinical geneticists tracking inherited disease may want to read both the DNA sequence and the methylation pattern together when standard genetic explanations fall short.
If a condition runs through a family without an obvious genetic cause, the answer may sit in a layer that standard sequencing tools never read.
The study is published in the journal Nature Genetics.
Image Credit: Art design by Michael Koldobskiy and Andrew Feinberg, illustration by Kate Zvorykina
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