Pea flowers of purple and white

14 August 2019

Interview with 

Xander Jones, University of Cambridge

PEA PLANT

Purple pea flower

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What does the ‘genetics’ mean to you? Maybe you’re an expert - or maybe you’re a newbie; someone who’s heard of genes, and DNA testing, and genetic diseases, but someone who’s not sure how all the pieces fit together. So let’s fix that, by going back to where it all started. This is a trip through time to meet a German monk called Gregor Mendel, who kick-started a brand new type of science. Phil Sansom went out in the garden to speak to plant scientist Xander Jones...

Xander - So we're in the Cambridge University Botanic Garden. Right now we're looking at a pea flower much like the ones that Mendel would have been working with when he was studying the rules of inheritance.

Phil - They’d really look like that? Because this is like... would you call this a blossom? And it's covered in purple flowers?

Xander - Yeah. it wouldn't be exactly like this because this is not a common pea that he would have been working with, but the flower shape is just like he would have seen.

Phil - You’ve got these weirdly-shaped purple petals, one of which is hooked around from underneath, and you're pulling them open like insect parts. And inside there's all these yellow little protrusion... what looks like threads, with yellow parts on the end.

Xander - And next to the yellow parts is one white part, and that’s actually the female tip.

Phil - These are all tiny.

Xander - They're very small. So Mendel was a monk in a monastery in what's now the Czech Republic. And he was interested in the rules of inheritance. So it was known that if two parents had certain traits that the offspring were likely to have similar traits. But the going theory at the time was that the traits would simply blend. That's understandable, because many traits do work like that; we often think of height like this. But Mendel had figured out that some traits didn't work like that, and set out to study it in a more rigourous way. He originally wanted to work on mice I’ve heard, but then his superior thought that might be a little too untoward, to be controlling which mice were mating with which mice. So they settled on peas. Which I think was a smart choice.

Phil - How many of these did he have in his monastery garden?

Xander - Hundreds. Even thousands. He quantified many thousands of plants in terms of their traits. So he picked out seven traits that had this certain characteristic. So I'm showing you this flower here. Sometimes there’ll be a purple-flowered pea; sometimes when you bred a purple flower with a white flower, you got only purple flowers.

Phil - Weird.

Xander - Really weird. And even weirder is in the next generation, all of a sudden white flowers would reappear. So a trait was sort of hidden. And he thought there was something about that that was interesting and important, and he was right. And so what he figured out was that if he took these yellow parts, the male parts, and he removed them before it self-pollinated...

Phil - Oh no, you're picking apart the flower...

Xander - Yeah.

Phil - Oh boy, you’ve taken all the parts off.

Xander - Well that one I messed up. So that shows you how good he had to get at this delicate operation. So I’ll take another flower and I’ll remove the male parts. This is called emasculating. So now we have a flower that only has the female part, the stigma, waiting for it to be pollinated.

Phil - And Mendel did this with his hands thousands of times?

Xander - He may have had more sophisticated tools like tweezers.

Phil - But still he had to have this level of care...

Xander - Thousands and thousands of times.

Phil - That's incredible.

Xander - It's a labour of love, I’m sure.

Phil -  It seems so simple but in theory what you've just done is crossbreed one flower with another, right?

Xander - Right, and if you knew the traits of the parents then you could track those traits into the next generations. So he didn't know anything about what the basis for those traits were, but he could see how the traits would come out in terms of the numbers in each generation. He had purple flowers that were true breeding and white flowers that were true breeding, he’d cross them, the next generation would be purple. After he selfed that generation, so the flowers that had the male parts and the female parts would do their own business, and then in the next generation he would see white flowers again - twenty-five percent of the time, on average - and seventy-five percent of the time he would see purple flowers.

Phil - You've got a purple-flower plant and a white-flower plant. You breed them. You get their kids.

Xander - They're all purple.

Phil - The kids are all purple. But the kids breed with themselves. And the grandkids you get, for every one white, you get three purple.

Xander - That's exactly right. Which isn't so much information, but he was able to surmise quite a lot from that. That 3:1, it means that there are two factors in the middle generation that are sorting out; either together, mixed, or separate.

Phil - Those are what we now call the two copies of each gene right?

Xander - Right.

Phil - Do you think for him in his garden with his thousands of pea plants, over and over again putting one into the other... what do you think was going through his mind?

Xander - Actually my hunch is that he knew he was on to something interesting and that he wanted to make a contribution. Just like all of us do!

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