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What Is a Gene — And Why Most Traits Aren’t Controlled by One - Leejohnston - 01-09-2026
TITLE:
What Is a Gene — And Why Most Traits Aren’t Controlled by One
ESSAY — THE LUMIN ARCHIVE
INTRODUCTION — THE MYTH OF “THE GENE FOR X”
People often ask questions like:
• Is there a gene for intelligence?
• Is there a gene for aggression?
• Is there a gene for obesity, creativity, or disease?
The question sounds reasonable — but it’s mostly wrong.
Modern biology has shown us something far more interesting:
Genes do not work like switches.
They work like probabilities.
To understand what a gene really is, and why most traits are not controlled by a single gene, we have to replace a simple picture with a deeper one — one that blends biology, statistics, and systems thinking.
SECTION 1 — WHAT A GENE ACTUALLY IS
At its simplest, a gene is a segment of DNA that contributes to the production of a functional molecule, usually a protein.
But that definition hides something crucial.
A gene does NOT directly create a trait.
Instead:
DNA → RNA → Protein → Cellular processes → Tissues → Organ systems → Traits
Each arrow represents layers of regulation, noise, interaction, and feedback.
A gene is not a command.
It is a parameter in a system.
This is why two people with the same gene variant can show very different outcomes.
SECTION 2 — ONE GENE, MANY EFFECTS (PLEIOTROPY)
Some genes influence multiple traits at once — a phenomenon called pleiotropy.
Example:
A single gene involved in connective tissue may affect:
• Height
• Joint flexibility
• Cardiovascular structure
• Vision
So even when a single gene has a strong effect, it rarely affects only one thing.
Biology reuses components.
Evolution builds by modification, not clean design.
This alone makes the idea of “a gene for X” misleading.
SECTION 3 — MANY GENES, ONE TRAIT (POLYGENIC TRAITS)
Most traits you care about are polygenic — controlled by many genes at once.
Examples:
• Height → hundreds to thousands of genetic variants
• Intelligence → thousands of small-effect variants
• Risk of diabetes → hundreds of interacting loci
• Personality traits → diffuse genetic influence + environment
Each gene contributes a tiny effect.
No single gene decides the outcome.
Think of it like this:
One gene might add +0.2
Another adds −0.1
Another adds +0.05
The trait emerges from the sum — not from any one part.
SECTION 4 — GENES AS PROBABILITY SHIFTERS
A powerful way to think about genes is this:
Genes shift probabilities, not destinies.
Having a certain genetic variant does not mean:
“You will have trait X.”
It means:
“The probability distribution of outcomes is shifted.”
Two people can have the same genotype and end up on different sides of the distribution due to:
• Environment
• Developmental randomness
• Epigenetic regulation
• Chance molecular events
This is why identical twins are similar — but not identical.
SECTION 5 — POPULATION GENETICS: WHERE THE MATH LIVES
When biologists study genes properly, they do not look at individuals.
They look at populations.
Key quantities include:
• Allele frequency (p)
• Mean fitness (w̄)
• Variance in fitness (σ²)
A simplified population rule looks like:
Change in allele frequency over time depends on how that allele changes average reproductive success relative to others.
In words:
Genes spread when they slightly improve survival or reproduction — on average.
Not because they “control” a trait.
SECTION 6 — WHY SINGLE-GENE DISEASES ARE RARE (AND SPECIAL)
Some diseases really are caused by single genes:
• Cystic fibrosis
• Huntington’s disease
• Sickle-cell disease
These are exceptions — not the rule.
They happen when:
• A gene is essential
• A mutation severely disrupts function
• Compensation is impossible
Most traits and diseases are nothing like this.
They exist on a spectrum of risk, not certainty.
SECTION 7 — ENVIRONMENT IS NOT OPTIONAL
Genes do nothing in isolation.
Nutrition, stress, toxins, social environment, and development all interact with genetic predispositions.
A classic example:
Height is highly heritable — but average height changed dramatically over the last century due to nutrition alone.
Same genes.
Different outcomes.
Genes load the dice.
Environment rolls them.
SECTION 8 — WHY “GENETIC DETERMINISM” FAILS
Genetic determinism sounds scientific — but it collapses under real data.
If genes fully determined traits:
• Identical twins would be identical
• Environment wouldn’t matter
• Variation would be minimal
Instead, biology shows:
• Robustness
• Redundancy
• Noise
• Adaptability
Life is not brittle.
It is statistical.
SECTION 9 — EVOLUTION RUNS ON VARIANCE, NOT CERTAINTY
Natural selection does not choose “the best gene.”
It filters distributions over time.
Small advantages compound.
Small disadvantages fade.
Chance always plays a role.
Evolution is a probabilistic algorithm, not a blueprint.
SECTION 10 — THE BIG IDEA
A gene is not:
• A switch
• A destiny
• A command
A gene is:
• A contributor
• A bias
• A probabilistic influence inside a complex system
Traits emerge from interactions — not instructions.
CONCLUSION — WHY THIS MATTERS
Understanding genes properly protects us from:
• Oversimplified science reporting
• Genetic fatalism
• Misuse of biology in social debates
And it reveals something deeper and more beautiful:
Life is not controlled by single causes.
It emerges from many small influences interacting over time.
Genes don’t decide who you are.
They shape the landscape in which you become.
— The Lumin Archive
Long-form Essay Series