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Comment to the reader.
In the previous entry I described what humanity at this time understands about how life began on Earth — the chemistry that produced the first replicating molecules, and the moment at which ordinary matter first acquired the property of making copies of itself.
This entry continues the How We Came About series. Once a molecule that copies itself exists, two questions follow immediately: what is the copy actually made of, and how does the information get faithfully transmitted from one generation of replicators to the next? This is the domain of genetics — the study of inheritance, of the molecule that carries it, of the small mistakes that introduce variation, and of the structures that turn that variation into the diversity of every living thing.
I write this entry in the spring of 2026 of the Common Era. The science of heredity is among the most rapidly developing fields in our era; substantial parts of what follows would have been unknown or only dimly suspected fifty years before, and parts of it are still being worked out as I write. I have set out the picture as it is broadly understood at this moment, noting where the picture is settled and where it is still being argued over.
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What carries the form of a parent into a child?
For most of human history, the answer was obvious only by what one could observe: children resemble their parents. They have a parent's eyes, a grandparent's nose, the shape of a great-grandfather's forehead. Some traits skip a generation. Some seem to come from neither side and surprise everyone. Whatever was being transmitted, it was being transmitted somehow — and yet for thousands of years the mechanism was completely unknown.
The solution turned out to be a molecule.
This entry describes what we now know about that molecule, about the rules by which the information it carries is passed on, about the small errors that creep in during copying, and about the way those errors — far from being a defect of the system — are the engine that drives every change in the living world.
The first person to make real progress on the mechanism of heredity was a monk named Gregor Mendel, working in the garden of an abbey in Brno, in what is now the Czech Republic, between 1856 and 1863. Mendel grew tens of thousands of pea plants. He counted the offspring of carefully arranged crosses, noting whether the seeds came out wrinkled or smooth, the flowers white or purple, the plants tall or short.
What he found, in the patterns of those numbers, was that the traits of his peas behaved as if they were carried by discrete particles — units that did not blend together when a tall plant was crossed with a short one, but instead were inherited intact, distributed to offspring in predictable mathematical ratios [1]. A tall × short cross did not, as common sense suggested, give rise to plants of intermediate height. The first generation was uniformly tall; the next generation reproduced the original short plants in roughly one-quarter of the offspring. The "short" instruction had not vanished. It had been carried, hidden, through the first generation, and re-emerged in the second.
Mendel published his results in 1866, in the proceedings of a small natural-history society. They were almost entirely ignored for the next thirty-four years. When they were rediscovered around 1900 and properly understood, they founded the modern science of heredity. The hereditary particles Mendel had inferred were eventually given a name: genes.
But what was a gene, physically? For another half-century, no one knew.
The answer was identified, in stages, between the 1940s and the 1950s.
In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty showed, by careful purification work on bacteria, that the substance which carried hereditary information was deoxyribonucleic acid, or DNA [2] — a molecule that had been known to chemists for decades but had been considered too uniform, too repetitive, to plausibly carry the diversity of life. Most biologists of the day expected the answer to be a protein instead. In 1953, James Watson and Francis Crick — drawing crucially on X-ray crystallography work by Rosalind Franklin and Maurice Wilkins — proposed the structure of DNA that has stood ever since [3].
DNA is two long, thin strands twisted around each other in a regular spiral. Each strand is a backbone of alternating sugar and phosphate units. Attached to each sugar, pointing inward toward the partner strand, is one of four small molecules called nitrogenous bases: adenine, cytosine, guanine, and thymine — abbreviated A, C, G, and T. Inside the helix, the bases pair up across the gap: A with T, C with G, always. The two strands are not identical; they are complementary. Where one strand reads ACGT, the other reads its mirror image: TGCA.
This complementarity is what makes inheritance possible. To copy a DNA molecule, the cell unwinds the helix, separates the two strands, and uses each one as a template — running along it base by base, slotting in the complementary partner each time. From one double helix, two are produced, each consisting of one old strand and one new one. Each daughter molecule is, to a very high degree of fidelity, identical to the parent.
The chemical composition of the four bases is the same in every living thing. A T-base in a worm is the same molecule as a T-base in a human. The only difference between species, and between individuals within a species, is the sequence in which those four bases appear along the strand. Life on Earth is written in a four-letter alphabet, and the differences between every organism that has ever lived are differences in spelling.
A typical human body cell contains two complete copies of the genome — about 6.2 billion base-pairs of DNA in total — distributed across forty-six chromosomes (twenty-three pairs). A single haploid set, such as that carried in a sperm or an egg cell, contains about 3.1 billion base-pairs across twenty-three chromosomes [4]. If the DNA from a single ordinary body cell were unwound and laid end to end, it would extend roughly two metres — folded, in life, to fit inside a cell nucleus only a few micrometres across. The human body, at adult size, is built of around thirty-seven trillion cells [6]; almost every one of them carries a complete copy of the same DNA.