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Sweet Peas and Punnetts

Sweet Peas and Punnetts

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Our story starts in the English county of Kent in 1884, in the elegant middle-class home of the Punnett family. Nine-year-old Reginald Crundall Punnett is grappling with his first bout of appendicitis.

He’s fascinated by the leeches that have been placed on his belly - a common treatment at the time – carefully noting how long each one takes to become fat and gorged on his blood.

Sent to rest in the quiet confines of his family’s library for a few hours a day after lunch, young Punnett skips his naps in favour of reading, furthering his interest in the delights of nature by beavering his way through forty volumes of the Naturalist’s Library by William Jardine, which his father had picked up at a sale.

The purchase wasn’t due to any particular interest in science – rather, Punnett senior had only bought the books because he liked the decorative bindings – but it was enough to set Reginald firmly on the road to becoming a scientist.

Punnett spent the rest of the summer collecting bird’s eggs and butterflies, visiting Brighton aquarium and eagerly devouring any nature books he could get his hands on, much to the bemusement of his bourgeois, church-going family and friends, who had no interest in science at all. All his hard work paid off, and he won a scholarship to Cambridge University to study medicine, grappling with yet another bout of appendicitis along the way.

Punnett quickly became fascinated with zoology, ditching medicine in favour of a career in research focusing on marine animals. By 1901, he had been elected a fellow at Gonville and Caius college in Cambridge (usually just known as Caius, pronounced ‘Keys’) and had also finally had enough of his dodgy appendix. He popped down to London to get it taken out for good, taking with him a specimen jar and preservative so he could dissect the offending organ at a later date.

While recovering he kept on reading and became intrigued by an exciting new idea from Austrian embryologist Leopold Schenk, suggesting that diet might have an influence on sex determination. Punnett was determined to investigate and wrote to pioneering geneticist William Bateson, who was busy carrying out breeding experiments with plants and animals, to see if he could collaborate. Bateson jumped at the chance, and so began Punnett’s transformation from zoologist to geneticist.  

It turned out that mouse geneticist Florence Dunham was already looking into the sex question, but there were plenty of other problems for Punnett to get his teeth into. Together with Bateson, his collaborator Edith Rebecca Saunders and Robert Heath Lock, who was curator of the Cambridge University Herbarium, Punnett set about trying to devise a way of describing and explaining all the different possible offspring from breeding sweet peas with various characteristics, based on Mendel’s laws on inheritance.

Gregor Mendel himself first put forward the idea of using some kind of square diagram to figure out the likely outcomes from a cross in his classic 1866 pea paper, although obviously his ideas were lost until they were rediscovered by Bateson and others at the turn of the 20th century, as we heard in episode 14. Punnett took this forward, coming up with the idea behind what we now call a Punnett square, which was popularised by Bateson.

At its most simple, a Punnett square is a simple square divided into four smaller boxes, describing the possible outcomes of crossing together two individuals with various combinations of dominant or recessive versions of a single gene, known as alleles. But this gets much more complicated when considering more possible parents and multiple traits.

The challenge of visually representing the outcome of genetic crosses also attracted the attention of Charles Darwin’s cousin Francis Galton, who in 1905 sent Bateson an elegant hand-coloured square capturing the 64 possible outcomes of crossing three different characteristics.

The diagram was so clear that Bateson and Punnett adopted the design immediately, with Punnett putting a version in the 1907 reprint of his best-selling book Mendelism, when it hadn’t appeared in the first edition in 1905. This marked the first time that such a square was published, forever linking it to Punnett’s name, although he never actually crediting Galton with coming up with that particular format in the first place.

The issue may have even attracted the attention of John Venn, who invented his eponymous diagram in 1880. Venn was also a fellow at Caius, and he and Punnett doubtlessly interacted at college - perhaps over dinner and port - but while his overlapping circles are pleasing, they’re not as useful in genetics as Punnett’s squares.

As time has passed, things have become much more complicated. Although the Punnet square might still be taught in school as a way of working out the chances of inheriting traits or diseases caused by obvious single gene faults, most characteristics in humans, animals and plants can’t be explained so easily. Modern genetics is more about statistics than squares, thanks to complex interactions between genes, their control switches, and the influence of the environment (known as epigenetics).

This was also a problem that Punnett grappled with later on in his career, playing an essential role in the development of one of the most important mathematical formulas in genetics, known as the Hardy-Weinberg equation, which explains the proportions of genetically different individuals in a population. Today, researchers use complex algorithms to figure out the interactions between thousands of regions of the genome and all kinds of traits and diseases, from diabetes and cancer to height, weight or even voting tendency.

Although Punnett didn’t directly name his square, having things named after oneself seems to be a recurring trait. Punnett himself has two species of sea worms named after him - Cerbratulus punnetti and Punnettia splendia.

And one of his 18th century farming relatives gave the family name to the small wooden boxes he designed to transport his soft fruits to the markets in London, which we still refer to as punnets – a fun science fact to enjoy while you’re tucking into a bowl of strawberries and cream this summer.

References and further reading:

Double (Helix) Trouble

Double (Helix) Trouble

Three is the magic number

Three is the magic number