Mendel’s Seven Traits: Why the Genes Were Found Late

An Austrian monk planted peas in a monastery garden, counted around 28,000 of them over seven years, and worked out the rules of inheritance. Then he published the result, and for thirty-five years almost nobody read it. Today his name is on the first page of every genetics textbook.

This post covers the seven traits he chose, what each one shows, and how the work was nearly lost.

Mendel spent his working life at St Thomas’s Abbey in Brno, where he kept a two-hectare experimental garden — Image: Unknown author, public domain, via Wikimedia Commons

Why the Garden Pea Was the Right Plant

Mendel did not pick peas at random. He needed a plant that would let him answer a specific question without losing control of the answer. Four properties made it work.

• A short life cycle. Several generations fit into a few years, so a seven-year study could actually cover several.
• Controlled pollination. The flowers open in a way that lets him close them and pollinate a plant with itself, or dust one by hand.
• Clear, binary traits. Every character he studied came in two shapes with nothing in between. No half-purple flowers.
• True-breeding lines. He started from plants that reproduced true to type generation after generation, so any change later was his doing.

That last point is where most accounts of Mendel go wrong. He did not just cross plants and look. He proved each line bred true first, and only then started.

The pods, the seeds, and the flowers are all visible at once in a single plant — Image: Bill Ebbesen, CC BY-SA 3.0, via Wikimedia Commons

The Seven Pairs of Contrasting Traits

Mendel settled on seven characters that he had reason to believe moved independently of one another. Each exists in two forms. He coined the words dominant and recessive to describe what he saw.

Each character has exactly two forms, with no intermediate state. That is what made the counting possible — Image: LadyofHats, CC0, via Wikimedia Commons

Trait	Dominant form	Recessive form	Letter
Seed shape	round	wrinkled	R / r
Seed color	yellow	green	Y / y
Pod shape	inflated	constricted	I / i
Unripe pod color	green	yellow	G / g
Flower color	purple	white	P / p
Flower position	axial	terminal	A / a
Stem length	tall	dwarf	T / t

Why the letters matter

Mendel never used the word gene. He called them factors, and he did use capital and lowercase letters to mark which form dominated. That notation is still standard a century and a half later.

For seed shape there are three possible combinations, not two. RR is round. rr is wrinkled. Rr is round, because R masks r. Two identical alleles is called homozygous; two different ones is called heterozygous.

Both homozygotes and the heterozygote look round. That is the whole trick of the first cross, and it is why the recessive form vanishes from view in the first generation without being gone.

What One Cross Shows

Take two true-breeding lines, one round-seeded and one wrinkled. Cross them by hand. That cross is the P generation, the parents.

Every seed in the first filial generation, F1, comes out round. Not most of them, not round on average. All of them. A student who had expected a blend gets an answer with no trace of one parent at all.

P round x wrinkled gives all-round F1, then a 3:1 split in F2 — Image: Sciencia58, CC0, via Wikimedia Commons

Now let the F1 plants pollinate themselves. Every one of them is Rr, so each produces equal numbers of R and r gametes. That is the segregation, and it is why the recessive form comes back.

In F2 the expected split is three round to one wrinkled. And the genotype ratio underneath is different: one RR, two Rr, one rr. Two different ratios, describing two different things. Most confusion about Mendel in school comes from running those two together.

The wrinkled minority matters more than the round majority. Those plants prove the r allele survived the F1 generation instead of being blended away.

Two traits at a time

Mendel also crossed plants differing in two characters at once. When the two genes sit on different chromosomes, they assort independently, and the F2 ratio becomes 9:3:3:1.

Nine to three to three to one, when the two genes are on different chromosomes — Image: Archive, CC BY-SA 4.0, via Wikimedia Commons

The condition matters more than the rule. Independent assortment only holds when the genes are on different chromosomes or far enough apart on the same one. Link them together and the ratio bends. Mendel had no idea why his rule worked, because nobody knew what a chromosome was doing in 1865.

Peas mostly pollinate themselves. Mendel closed flowers to force the cross he wanted — Image: Rasbak, CC BY-SA 3.0, via Wikimedia Commons

Thirty-Five Years of Silence

Mendel presented his paper to the Brno Natural History Society on 8 February and again on 8 March 1865. It was published in 1866. Over the next thirty-five years it was cited about three times.

The reason is not that the work was wrong. It is that it did not fit what nineteenth-century biologists expected. They were looking at traits that blend smoothly, like height, and averaging them. Mendel had deliberately chosen traits with no in-between, which looked like a peculiarity of his plant rather than a general law.

Even Mendel was unsure. He told his friend Gustav von Niessl that his time would come. Charles Darwin, working on inheritance at the same moment, never knew the paper existed.

Then in 1900 three European scientists arrived at the same result independently and went looking for why. Hugo de Vries and Carl Correns both found Mendel and credited him. Erich von Tschermak is no longer counted, because he never understood what the laws said.

De Vries and Correns published their rediscovery within two months of each other in 1900 — Image: Thérèse Schwartze, public domain, via Wikimedia Commons

Correns pointed out Mendel’s priority after reading de Vries’ paper and realising he did not have it — Image: Unknown author, public domain, via Wikimedia Commons

William Bateson then supplied the vocabulary, coining the word genetics and the term allele. Thomas Hunt Morgan connected Mendel to the chromosome theory in 1915. Ronald Fisher joined Mendel to natural selection in his 1930 book, which is what turned two separate ideas into modern evolutionary biology.

The word genetics is Bateson’s, as is allele — Image: Unknown author, public domain, via Wikimedia Commons

Morgan put Mendel on chromosomes, creating classical genetics — Image: Unknown author, public domain, via Wikimedia Commons

The Genes, Found a Century Late

Mendel said there were seven factors. For over a century nobody knew what they were physically. The search ended in 2025, when the last three were identified in the pea genome.

• PsSBE1 — wrinkled seeds against the wild-type round. The mutation is an insertion.
• PsSGR — yellow against green seeds.
• PsbHLH — white against purple flowers, caused by a deletion.
• PsGA3ox1 — dwarf against tall.
• PsChlG — green against yellow pods.
• PsCLE41 — constricted against inflated pods.
• PsCIK2/3 — terminal against axial flower position.

Two details are worth sitting with. The wrinkled-seed gene is broken by the insertion of a piece of other DNA, not by a substitution. And the gene that makes peas tall is a growth-hormone gene, which is why dwarf plants are not simply smaller versions of tall ones.

The data that worried Fisher

In 1936 Ronald Fisher rebuilt Mendel’s numbers and found something odd. The ratios were not just close to 3:1. They were implausibly, consistently close, closer than honest counting should manage. He suggested the data had been adjusted.

This is still called the Mendelian paradox, and it has never been settled. Daniel Hartl and Daniel Fairbanks reviewed the evidence and concluded there is no basis for the charge.

Their argument is that Mendel had to break through the assumptions of his audience to get the work published at all. The honest position is that nobody knows, and nobody accuses Mendel of anything.

The Rules in One Table

Rule	What it says	Limit on it
Dominance	One allele masks another in the heterozygote	Relative between two alleles, not a fixed label
Segregation	The two alleles separate when gametes form	Holds for single-gene traits
Independent assortment	Different genes do not affect each other	Only when the genes are unlinked
Ratio, one trait	3 dominant to 1 recessive in F2	A genotype ratio of 1:2:1 sits underneath it
Ratio, two traits	9:3:3:1 in F2	Same unlinked condition

Key Takeaways

• Mendel was an Augustinian friar at Brno who ran a two-hectare monastery garden and tested about 28,000 pea plants between 1856 and 1863.
• He chose traits with only two forms and started from lines he had proved bred true. Both choices were deliberate.
• All F1 offspring looked like one parent. That was not a blend, and it is what the word dominance describes.
• The recessive form disappearing in F1 and returning in F2 at one in four is the evidence that alleles separate rather than mix.
• The 3:1 phenotypic ratio and the 1:2:1 genotypic ratio are different things describing the same cross.
• Independent assortment only applies to genes that are not linked on the same chromosome.
• The work was published in 1866, cited about three times in thirty-five years, and rediscovered in 1900 by de Vries and Correns.
• Mendel never used the words gene, allele or heterozygote. Bateson supplied those, along with the word genetics.
• The seven genes were finally identified in 2025. The wrinkled-seed gene is broken by an insertion of other DNA.

Frequently Asked Questions

Why did nobody notice Mendel for 35 years?

His traits were binary where most inheritance of his day looked continuous and blended. That made his results look like a quirk of peas rather than a law. Darwin, working on the same question, never knew the paper existed.

Did Mendel really discover DNA or genes?

No. He established the rules of inheritance and proposed that something particulate was passed down. He did not know what that something was. The physical basis took until the chromosome theory and later DNA work, most of it after his death.

Is the 3:1 ratio exact?

No. It is an expectation, and real counts scatter around it. Mendel used thousands of plants precisely so that chance would average out. In a small cross you might see five round and one wrinkled, or all six round.

Why did Mendel pick yellow as dominant for seeds and green for pods?

Dominance is not a property of a color. It is a relationship between two alleles of one gene. Yellow happens to dominate green in the seed gene, while green dominates yellow in the pod gene. The same color goes the other way in the two cases.

Do Mendel’s rules always apply?

No. Many traits are controlled by several genes at once, and then they do not sort cleanly at all. Height and skin color are the standard examples. Biologists call those exceptions non-Mendelian inheritance.

Did Mendel lie about his data?

Nobody knows. Ronald Fisher argued in 1936 that the numbers were suspiciously exact. Later researchers including Hartl and Fairbanks concluded there is no evidence of falsification and that his results can be accounted for. The question is genuinely open.

Sources: Wikipedia articles on Gregor Mendel, Mendelian inheritance, Pisum sativum, dominance (genetics), allele, law of segregation, law of independent assortment and Thomas Hunt Morgan. Images: Wikimedia Commons, with authors and licenses noted in each caption.