MENDEL'S PEAS
Where the rules came from
Gregor Mendel ran thousands of experiments with pea plants in the 19th century and introduced the idea of dominant and recessive inheritance.
See the numbersStart
WHAT GENETICS ACTUALLY SAYS
Not every trait
is a pea.
Mendel's peas made inheritance look simple.
Classroom charts still list tongue rolling, earlobes and eye colour as one-gene traits. Genetics teaching resources now say most of these are shaped by many genes, and sometimes by learning too.
MANY GENESmost of the time
Check a traitKeep in mind
- Few traits are one gene
- Families still share a lot
- Surprises are normal
- Simple rules, messy people
THE SHORT ANSWER
Some traits follow rules.
Most are a blend.
Mendel found clean patterns in peas.
Many human traits are more tangled.
EYE COLOUR
Not one gene after all
Brown was once taught as simply dominant to blue. MedlinePlus says multiple genes are involved, and two blue-eyed parents can, uncommonly, have a brown-eyed child.
Check a trait
PARTY TRICKS
Tongues, thumbs and earlobes
Tongue rolling can be learned, and no published study backs the one-gene story for earlobes. Genes play a part, but not the simple part the charts claim.
Spot a mythHOW PREDICTABLE IS IT?
Pick a trait,
see how it passes on.
Choose a trait to see how closely it follows a simple single-gene pattern. A full bar means it is passed on in a predictable way; a short bar means many genes, the environment or both are involved.
Based on the University of Utah Genetic Science Learning Center and MedlinePlus Genetics. General information about traits, not a test of anyone's family.
TWO NUMBERS WORTH KNOWING
Genes matter a lot,
but rarely alone.
One figure for a trait made of many genes,
one for a trait genes do not fully decide.
≈ 80%
Share of height down to inherited DNA variation
Scientists estimate about 80 percent of a person's height comes from inherited DNA variations. More than 700 variants have been found, each with a modest effect, which is why siblings can differ.
HEIGHT · MEDLINEPLUS
70%
Identical twins who share the trait
Identical twins share their genes, yet only 70% of them match on tongue rolling. In 1940 Alfred Sturtevant observed that people can learn the trick as they get older.
TONGUE ROLLING · UTAH
Handedness is another example: the Utah centre says it is shaped by at least 30 and perhaps 100 genes, each with a small effect, plus culture. About 10% of people are left-handed overall.
FOUR WAYS TO SPOT A MYTH
Read the family chart
with care.
Old textbook tables are still shared online. A few questions help sort a real single-gene trait from a tidy story.
Step 1
Is the trait either-or?
Earlobes, hairlines and hair texture come in in-between forms. Continuous traits usually mean many genes.
Step 2
Can it be learned or changed?
Tongue rolling can be learned and sun brings out more freckles. The environment is part of the picture.
Step 3
Do twins always match?
If identical twins sometimes differ, genes are not the whole explanation.
Step 4
Look for the study
For earlobes and widow's peak, the Utah centre says no study supports the one-gene claim often repeated.
WHEN THE RULES DO HOLD
Single-gene patterns
are real, just rarer.
Some conditions do follow Mendel's patterns closely. MedlinePlus lists autosomal dominant, autosomal recessive, X-linked and mitochondrial inheritance, among others. In recessive conditions such as cystic fibrosis, both parents usually carry one altered copy without showing signs.
Questions about a condition in your own family belong with a GP or a genetics service. This page is about everyday traits, and the point is simple: a tidy chart is a starting point, not a verdict.
Questions
Questions on the way
Can two blue-eyed parents have a brown-eyed child?
Yes, though it is uncommon. MedlinePlus says the old single-gene model, with brown dominant to blue, was too simple, because several genes affect eye colour.
Is tongue rolling genetic?
Partly at most. People can learn it as they get older, and only 70% of identical twins share it, according to the University of Utah Genetic Science Learning Center.
Are attached earlobes recessive?
Some sources say so, but the Utah centre says no published studies support a single-gene explanation. Earlobe shape is inherited but probably involves many genes.
Why is colour blindness more common in boys?
Red-green colour blindness is caused by a gene on the X chromosome. Boys have one X, so one altered copy is enough, while girls need two. The Utah centre gives 1 in 12 boys and 1 in 250 girls.
What did Mendel actually discover?
Working with pea plants in the 19th century, Gregor Mendel found general patterns in how traits such as colour and shape pass from parents to offspring, introducing dominant and recessive inheritance.
Destination
Where the facts come from
- University of Utah Genetic Science Learning Center: Observable Human Characteristics
- MedlinePlus Genetics: Is eye color determined by genetics?
- MedlinePlus Genetics: Is height determined by genetics?
- MedlinePlus Genetics: What are the different ways a genetic condition can be inherited?
- National Human Genome Research Institute glossary: Mendelian inheritance
- NHS: Colour vision deficiency (colour blindness)
Updated