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The Genetics of Gregor Mendel: A Game of Seeds!

The Genetics of Gregor Mendel: A Game of Seeds! Image: Pixabay

It wasn't that long ago that the notion of inheritance of characteristics through genes was somewhat fanciful. As Kirsty Martin explains, one green-fingered scientist's work cultivated the bloom in genetics we see around us today. Gregor Mendel is the focus in the latest in a series of blog posts for Glasgow City of Science on the theme of 'Whose Shoulders Are These Anyway?'

 

In our 21st century world of technology the ideas of genetics have become an integral theme in our day to day lives, and even our pop-culture. We expect a DNA analysis to catch out the bad guy in many a procedural crime show; and Joffrey Baratheon’s blonde head was a red flag to many in the developing drama of ‘Game of Thrones’!

It is easy to forget that only 100 years ago the idea of inheritance of characteristics through genes was both new and radical. We now assiduously look for genes to explain everything, from mental health conditions to sexuality to sleep patterns; but prior the 20th century the mechanism of inheritance was one of the abiding mysteries of science.

This shift in our understanding of our very nature is the emergent result of the work of hundreds of researchers, from students to professors. Glasgow has played its role in this world changing saga, which has, excitingly, recently been documented by the Wellcome Trust in their “Codebreakers” resource. Notably, among the luminaries whose papers have been digitised for that archive are Guido Pontecorvo, the first Professor of Genetics at the University of Glasgow, and Glasgow native Malcom Ferguson-Smith, who was the first Burton Professor of Medical Genetics at that same institute.

There is, however, a notable gap in the resource; one man whose work is generally accepted to be the starting point of our genetic revolution, and whose papers were, sadly, unavailable. The origin of genetics as a science was, arguably, in the monastery of St Thomas (below), in the town of Brno (now in the Czech Republic), in the 1860’s. This was where Gregor Mendel, an Augustinian friar from a modest farming background, spent his days cheerfully cultivating the pea plants that would change the world. The basic rules of ‘particulate inheritance’ which he uncovered, and which we now know as Mendelian Genetics, are one of the foundation stones of an entire field of study.

Mendel Museum and monastery of St Thomas, Brno

Born Johann Mendel, the future Brother Gregor was the only son of Anton and Rosine, and brother to Veronika and Theresia. Our hero learned the skills that would bring about his greatest discoveries – those of a gardener – as a child on his parents farm. The formal education of his scientific method was instilled, with some struggle owing to health and financial difficulties, by attendance at the Universities of Olomouc and Vienna, and included courses taught by Christian Doppler (of the Doppler Effect!).

Some of his earlier university education was made possible by his sister Theresia, who gifted a part of her dowry to pay for her brother’s studies. Its continuation was allowed when Johann joined the Augustinian Order (where he took the name Gregor), which relieved him of the financial burden of further learning. The Abbot of St Thomas at the time shared an interest in the problems of heredity in sheep with Johann Nestler, one of Mendel’s university teachers, and so encouraged and personally sponsored Brother Gregor’s continued education and experimentation. In deference to their bishop’s religious sensibilities, however, the Abbot had Mendel give up his original aim of breeding mice to study inherited traits. The pea plants appear to have been an agreeable compromise; luckily, according to Mendel, “the bishop did not understand that plants also have sex.”

Gregor Mendel

Over the years Mendel’s carefully regulated crossbreeding of plants and apparently meticulous recordkeeping allowed him to observe something remarkable; the ‘characters’ of his plants (flower colour, seed shape, seed colour and so on) were passed onto their offspring according to strict rules that he could define mathematically.

He started with plants that ‘bred true’ for on form of a character (so self-fertilisation of the plants always generated the same form – for example white flowers or purple flowers – down the generations). He then crossed true-breeding plants of the two different forms of the character (purple flower with white flower). This invariably produced a generation of plants (called the F1 generation) with only one form of the trait, however many offspring he generated. And this held true for the other characters he recorded – there was always on form that dominated the other when true-breeding plants of different forms were crossed. This was a dramatic and important observation, given the long running debate around inherited characteristics and the ‘blending’ of parental characteristics.

But Mendel’s experiments and observations did not stop there. He pushed the theory of dominance further, predicting that in the next generation of plants the white flowers would re-occur (as many a gardener could have told him), but only in a quarter of plants. He reasoned that each seedling must get one piece of flower-colour information from each parent, that the purple was dominant, but that if two white-flower instructions came back together in a future plant, white flowers would result. This idea was best conveyed visually in 1905 by Reginald C. Punnett in his book “Mendelsim”: the Punnett Square, as seen below, has become a key tool of geneticists throughout the years!

Punnett Square

But let’s get back to Mendel.

When he crossed his F1 plants (whose parents had been of two different true-breeding forms of a trait) with themselves, their offspring (the ‘grand-seedlings’ of the original true-breeders, if you will, known as the F2 generation) were a mixture of those original forms. This meant that the information for white flowers was still present in the purple-flowered second generation, even though all the flowers were purple. But, importantly, the form that was seen in the intermediate generation always outnumbered the recurring grand-parental form by a 3:1 ratio. Not content with this information alone, Mendel bred a fourth generation of plants by self-fertilization of the F2 plants (this new generation being designated… you guessed it, F3!). And at this point it gets really neat! Firstly, the white flowered plants bred true for white flowers; this branch of the family tree had lost the ‘purple’ information. However, only 1 in 3 of the purple flowered F2 plants bred true; the rest behaved like the F1 plants, generating the 3:1 ratio of purple to white. These results were exactly what Mendel had predicted from his hypothesis that every plant had two instructions for the trait, one from each parent, and that for each trait one form was dominant.

This is all pretty cool. But it gets even better when you remember that Mendel wasn’t just keeping track of the flower colour of his plants. He also looked at a number of other characteristics – all of which followed the same basic rules. On top of that, when looking at multiple traits in the same experiment he could see that different traits followed this pattern independently of one another. In other words, if he started with two plants each of which bred true for different forms of two individual traits (for example seed colour and flower colour), and generated an F1 that demonstrated only one dominant form of each trait, the F2 generation would have a mixture of plants that included every possible combination of traits, including ones with dominant coloured flowers but recessive coloured seeds! This is the idea of ‘particulate inheritance’ that was the most critical outcome of Mendel’s work; each trait is inherited independently of the others.

All this was reported in one paper in the “Proceedings of the Natural History Society of Brünn” in 1865. It was some decades before it was brought to light as a fundamental mechanism by which Darwin’s theory of natural selection could function, and finally linked to both Meischer’s DNA and the ‘coloured bodies’, or chromosomes, that Walther Flemming identified in the nucleus of cells in the late nineteenth century. It is one of the great tragedies of science that Gregor Mendel’s personal papers were burned upon his death, leaving us to wonder what else this remarkable monk may have discovered from his other line of experiments - in beekeeping!

 

Kirsty Martin graduated in biochemistry from the University of Glasgow in 2006, moved to Dundee to pursue her PhD in cell signalling at the MRC Unit there. She continued her scientific tour of Scotland with a post-doc in advanced imaging techniques at the newly founded IB3 institute at Heriot Watt University before returning to Glasgow where she's currently combining these skill sets working at the Beatson Institute for Cancer Research.

Outside of the lab Kirsty's an avid reader of classic, historical and fantasy literature; she enjoys puzzle and adventure games; and her geek status is cemented by a love of fibrecrafts, particularly knitting and crochet. She's always excited to find things that combine these varied professional and personal interests!

You can read other blog posts in this series of "Whose Shoulders Are These Anyway" posts from Kirsty on 'Living Liebig’s laboratory legacy', 'What’s In Your DNA, Friedrich Miescher?, 'George's Marvellous Memorial', 'Dazzled by the Dark Lady' and 'Antonj van Leeuwenhoek: Appreciating the Little Things…'.

 

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