In 1866, a monk from an Augustinian monastery in Brno published a paper about peas in an obscure local journal. Nobody paid attention. The author died without ever knowing he had discovered the laws on which all of modern biology would one day rest. His name was Gregor Mendel — and his story is about how the most important discoveries sometimes come from the most unexpected places.
Practical genetics is as old as agriculture. Sumerian tablets four and a half thousand years old contain records of breeding horses for different qualities. Egyptian agronomist-priests at the temples of Karnak selected wheat seeds for yield and wrote the results on papyrus. This was heredity without understanding the mechanism — like controlling fire without knowing the chemistry of combustion.
Hippocrates proposed “pangenesis”: every part of the body releases tiny particles that are passed to offspring. Aristotle objected: then why do children sometimes resemble grandparents they never knew? He proposed his own theory — form comes from the father, matter from the mother. Both theories were wrong. But they asked questions that required another two thousand years to answer.
Avicenna in the eleventh century wrote about “familial predispositions” and documented haemophilia in brothers from one family — without understanding the mechanism, but clearly recording the pattern. He insisted on collecting the medical history of three generations before making a diagnosis. This was, in essence, a health genealogy — the very practice we are reviving today.
For thousands of years humanity applied genetics in practice — in animal husbandry, plant cultivation, medicine — without the slightest idea how it worked. Mendel was the first to ask “why” — and to get an answer.
Gregor Mendel was born in 1822 into a Silesian peasant family. The monastery in Brno was for him not a retreat from science but, on the contrary, the only way to get an education: Abbot Cyrill Napp sent him to study at the University of Vienna. There, Mendel failed the state examination for a teaching certificate twice — and returned to the monastery to grow peas.
Over eight years — from 1856 to 1863 — he cultivated around 28,000 pea plants and recorded the results of crosses across seven traits: seed colour, pod shape, plant height and others. He crossed pure lines, counted offspring, noted ratios. And he discovered something no one had seen before: traits are inherited in discrete units, in precise numerical ratios.
Mendel’s three laws: first-generation hybrids are uniform (all inherit the dominant trait); in the next generation traits segregate in a 3:1 ratio; and different traits are inherited independently of one another. He denoted these patterns with letters — A for the dominant allele, a for the recessive. That notation has survived to the present day almost unchanged.
In 1866 he published his paper “Experiments on Plant Hybrids” in the Proceedings of the Brno Natural History Society. The journal was distributed to 120 scientific libraries around the world. Mendel received one response — from the botanist Karl von Nägeli, who advised him to continue his work with hawkweed. Hawkweed reproduces apomictically — without normal sexual reproduction. Mendel’s laws do not apply to it. He spent years going nowhere.
Mendel died in 1884 as abbot of the monastery — embroiled in administrative conflicts with the Austro-Hungarian government over monastic taxes. His scientific notebooks were burned by his successor. Only the published paper and letters to Nägeli survived.
In 1900, three biologists — de Vries, Correns and von Tschermak — independently rediscovered his laws while conducting their own experiments. All three, checking the literature, found Mendel’s paper. And all three honestly cited him. Mendel received his recognition posthumously — and became the founder of a science that would, fifty years later, transform medicine.
Thomas Hunt Morgan ordered fruit flies — Drosophila melanogaster — for his laboratory in 1908. The reason was prosaic: they are cheap, breed quickly (a generation every two weeks) and have just four pairs of chromosomes. He planned to test Mendel’s laws on animals.
In 1910, among hundreds of red-eyed flies, one appeared with white eyes. Morgan crossed it with red-eyed females and found that white eyes were passed only to males. This meant the white-eye gene was located on the X chromosome. The first evidence that a specific gene occupies a specific chromosome.
His student Alfred Sturtevant, in 1913 — on his own initiative, in a single night — built the first genetic map of a chromosome: he showed that the more frequently two genes are inherited together, the closer together they sit. He introduced a unit of genetic distance — the centimorgan (cM, equivalent to 1% recombination frequency). That unit is still in use today.
In 1933, Morgan was awarded the Nobel Prize in Physiology or Medicine. At the banquet he proposed a toast to the fruit fly.
By the 1950s it was known that the carrier of hereditary information was DNA. This had been shown in 1944 by Oswald Avery: he proved that it was DNA, not protein as previously believed, that transforms non-pathogenic bacteria into pathogenic ones. One question remained: what was the structure of this molecule?
At King’s College London, Rosalind Franklin was producing X-ray crystallography images of crystallised DNA. On 6 May 1952 she produced Image 51 — a photograph that would later be called “one of the most beautiful X-ray images in the history of science.” It clearly showed the cross-shaped diffraction pattern characteristic of a double helix.
Franklin’s colleague Maurice Wilkins, without her knowledge, showed this image to James Watson. Watson and Francis Crick at Cambridge used Franklin’s data — and in April 1953 published a two-page paper in Nature proposing the double helix model of DNA. Franklin’s paper containing the experimental evidence was published in the same issue of the journal — as a supporting article.
Watson, Crick and Wilkins received the Nobel Prize in 1962. Rosalind Franklin died of ovarian cancer in 1958 — aged 37. The Nobel Prize is not awarded posthumously.
The double helix structure explained several things at once: how DNA is copied (complementary strands separate, each serving as a template for a new one), how information is stored (the sequence of bases) and why mutations arise (errors during copying). It was a structure that explained everything.
After Watson and Crick, the central question remained: how exactly does the sequence of DNA nucleotides determine the sequence of amino acids in a protein? This was the puzzle of the “genetic code.”
In 1961, Marshall Nirenberg and Johann Matthaei conducted a simple but brilliant experiment. They synthesised an artificial RNA made entirely of uridines (UUUUUU…) and introduced it into a cell-free extract of E. coli. The result: a protein made entirely of phenylalanine was produced. UUU = phenylalanine. The first decoded codon.
By 1966 the entire genetic code had been deciphered. 64 codons encode 20 amino acids plus three stop signals. The code is degenerate: most amino acids are specified by more than one codon. The code is essentially universal: it is the same in bacteria, fungi, plants, animals and humans. This fact is among the strongest evidence for the common origin of all life.
In 1987, Japanese scientist Yoshizumi Ishino discovered strange repeating sequences in the E. coli genome, separated by unique insertions. He published this as a curious anomaly — without any hypothesis about function.
It took nearly twenty years before Spanish biologist Francisco Mojica understood: this was the immune memory of bacteria. The unique insertions between the repeats were fragments of DNA from viruses that had once attacked the cell. The bacterium had “memorised” the enemy and stored a sample for recognition during future infections.
In 2012, Emmanuelle Charpentier and Jennifer Doudna published a paper in Science that turned biology upside down: they had adapted the CRISPR-Cas9 system to work with any DNA in any cell. One needs to create a guide RNA complementary to the desired location — and the Cas9 protein will cut precisely there. This was fundamentally simpler, cheaper and more precise than all previous methods of genome editing.
In 2020, Charpentier and Doudna received the Nobel Prize in Chemistry. It was a record-short interval from publication to Nobel — eight years.
Module 2 (Donor Selection & Genetics) explains how Extended Carrier Screening (ECS) works in practice and what to do with its results. Module 5 (IVF & Embryo Expertise) covers modern standards for preimplantation genetic testing. Both are available free in the Learn section.
one of two or more forms of the same gene. Denoted by a capital letter (dominant) or lowercase letter (recessive) in the notation introduced by Mendel.
a three-letter “word” of the genetic code: a sequence of three nucleotides that encodes one amino acid or a stop signal.
sequencing technologies that read billions of nucleotides in parallel within a few days. The foundation of modern medical genetics.
a molecular genome editing system adapted from bacterial immunity. Allows precise changes to be made to the genome of any cell.
analysis of embryo DNA before transfer to the uterus during IVF. Allows selection of embryos free from chromosomal abnormalities or specific mutations.