Mendel: Traits Come in Units
Inheritance is not paint mixing. Traits are carried as discrete units that can hide for a generation and reappear intact.
Builds on: 2.1 DNA: Four Letters, One Molecule2.4 Mutations: When the Copy Is Wrong
The blending problem
Before 1865 the common assumption was that inheritance blends: cross a tall plant with a short one and get medium offspring, like mixing paint. It is intuitive, and it is fatal to Darwin’s theory — blending halves variation every generation, so within a few dozen generations everything is uniform and selection has nothing left to act on. Darwin knew this was a problem and never solved it.
The solution had already been published, by an Augustinian friar counting peas in a monastery garden, and was ignored for thirty-five years.
What Gregor Mendel did
Mendel picked pea plants with clean either/or traits — tall or short, round or wrinkled, yellow or green — and, crucially, counted his offspring. Thousands of them. Biology at the time described; Mendel measured.
Crossing pure tall with pure short, he got no medium plants. The first generation was all tall. Shortness had vanished. Then he let those plants self-pollinate, and shortness came back — in almost exactly one quarter of the offspring.
A trait that disappears completely and returns intact cannot have been blended. It was carried, unchanged and hidden, by the first generation. Inheritance is particulate.
The vocabulary
- Gene — a stretch of DNA specifying a trait. Allele — one particular version of it (tall or short).
- Every organism carries two alleles per gene, one from each parent.
- Dominant (written capital, A) shows whenever present; recessive (lowercase, a) shows only when both copies are recessive.
- Genotype — the alleles you carry (AA, Aa, aa). Phenotype — what you can actually observe.
- Homozygous — two matching alleles. Heterozygous — one of each, a carrier of the recessive.
AA and Aa look identical. That single fact explains almost every puzzle in family genetics — including two brown-eyed parents having a blue-eyed child, and why cystic fibrosis can appear in a family with no history of it.
The Punnett square
Cross two heterozygotes (Aa × Aa). Each parent gives A or a with equal probability, so the four equally likely combinations are AA, Aa, aA and aa:
Note that these are probabilities, not guarantees. Four offspring from an Aa × Aa cross will not reliably be 3:1 any more than four coin flips give exactly two heads. Mendel’s ratios emerged because he counted thousands. Every genetic counsellor has to explain this: a 1-in-4 risk applies afresh to each child, and does not become 0-in-4 because the first three were unaffected.
He knew nothing of DNA, chromosomes or meiosis — none had been described. He inferred discrete hereditary units purely from ratios in counted offspring. His paper sat unread until 1900, when three researchers independently rediscovered the same rules and found he had beaten them by a generation. It is one of the best arguments in science for counting things.
⚗️ Lab — The Punnett Square
Pick two parents and read the offspring straight off the grid.
- Cross Aa × Aa — the classic 3:1. One in four offspring shows the recessive trait.
- Now cross AA × aa. Every offspring is Aa: the recessive vanishes entirely for a generation.
- Find the only cross where the recessive appears in half the offspring.
Work it by hand — 0 / 2
No multiple choice here: compute the value and type it. Suffixes like 2.5k or 20m are understood; answers within ±2% count.