Pink-flowered peas (Alderman x Salmon Flowered)

It was more than a decade ago that I made a cross between Alderman and Salmon Flowered, two heritage peas, with the vague dream of producing a good tasting culinary pea with pink flowers. But as my other three major pea-breeding projects took up more and more of my time, it kind of went by the wayside.

Just to recap: Alderman is a tall, elegant shelling pea with white flowers, introduced in 1891. Its flavour is outstanding, which is probably why it's one of the only Victorian tall peas which is still commonly available today. It's far and away my favourite shelling pea, and while there are a few others which can rival it for flavour when they're young, Alderman stays exquisitely delicious even when the peas are at full size and maturity, and I haven't found anything else that can match it. It has large, well-filled pods on large plants, its only disadvantage (if you consider it a disadvantage) is that it's quite late maturing. Salmon Flowered is a real rarity, whose seed I got from the Heritage Seed Library many years ago. It's an umbellatum-type pea, which means it has a heavily fasciated (thickened) stem with all the flower buds borne in a great clump at the top. The flowers bloom more or less all at once and the pods form in a big clump, sticking out in all directions. There were a few of these varieties around in the 19th century but they're no longer commercially available – and in terms of flavour and yield they can't really compete with modern varieties. But Salmon Flowered (not its real name, which has been lost*) has really beautiful and unusual bicolour pink flowers which I haven't seen in any 'normal' pea at all. The wing petals are a peachy salmon pink and the standard is a very pale blush pink.

*A Swedish heritage pea called Rosakrone is now available from Real Seeds and is very, very similar to Salmon Flowered. I grew some Rosakrone this year to see just how similar it was, and while I'd say it's not absolutely identical, it is similar enough that they're most likely different stocks of the same original variety. The pink flowers are very much the same colour.

So the purpose of hybridising Alderman with Salmon Flowered was to see if I could breed the pink flower trait into a crop of otherwise normal garden peas – using Alderman as the benchmark because of its exceptional flavour.

Back in 2010 I grew out the F1 seeds from my Alderman x Salmon Flowered cross and wrote about it here. The F1 plants all had bicolour purple flowers, which might seem like an absolutely bizarre thing to get from a cross between a white flowered and a pink flowered pea, but actually it's what I'd expected. Purple bicolour is the ancestral default flower colour for peas (not just culinary peas but also the sweet pea Lathyrus odoratus) and the only reason why most garden peas DON'T have purple flowers is that they've all been bred to have a recessive gene which suppresses the production of anthocyanin pigment in the plant. I went into some detail about that in my original post about this cross, so I won't repeat it all here, but suffice to say that Alderman's pure snow white blossoms are not due to any 'white flower' gene as such, they're caused by the presence of this gene which switches off the expression of purple colour so that the flowers are white by default. Salmon Flowered doesn't carry this colour-suppressing gene – it can't do, or it wouldn't have pink flowers – so when you make a cross between a variety which has the pigment-suppressing gene and one which doesn't, the F1 generation will default to the dominant condition – which is for colour to be expressed. Being the dominant ancestral trait, purple flowers prevail. But on a genetic level, the recessive half of the gene pair, which forces the flowers to turn white, is still there and ready to be passed on to a large proportion of the F2 offspring.

I only have a modest sized garden and very limited free time so the focus on my breeding for coloured pods (edible fibreless coloured pods at that) took up more attention and space for the next few years and the Alderman x Salmon Flowered went by the wayside. I had the bag of F2 seed which I produced in 2010, but hadn't sown them. So this 2019 crop was another "back from the dead" miracle story.

I found the F2 seeds in a box, and thought about how nice it would be to work on breeding for flower colour for a change, rather than pesky pod colour. But seeds from 2010 were only fit for the bin, surely? I had to do a quick test though rather than lob them straight out, so they went into a tray of water, alongside some other decrepit sideline projects.

I don't normally soak pea seeds or recommend soaking pea seeds. I was doing it here because it was just a germination test on 9-year-old seeds, and they were not expected to sprout. However, almost all of them did.

This goes against everything I've been taught about pea seeds. 1-2 years is commonly given as a pea seed lifespan. Some people report them lasting a bit longer if you store them in the fridge or the freezer. But mine hadn't had any special storage conditions. They were in plastic bags inside a cat food box on top of my bookcase. The 16 seeds (of quite diverse size and colour, being F2s) in the top right of the photo above became 16 sprightly little plants in a frame in the garden.

I didn't see any loss of vigour from the seed being nine years old. In fact one of the plants turned into the most productive pea I've ever grown, producing large numbers of pods on a multitude of sideshoots. That one will be covered in another blog post later, along with a couple of other interesting things which emerged from this growout.

I labelled the plants individually and numbered them from 1 to 16, and took notes on them as they grew, paying particular attention to the colour and density of the axillary pigmentation (the pinky purple splodges where the leaves join the stem) as I've increasingly noticed a significant correlation between axil splodges and the eventual flower colour and/or pod colour.

Well, I already knew that the F1 had produced purple flowers, so what was I expecting to get from the F2? In a sample size of only 16, I wouldn't expect to get perfect Mendelian ratios for anything, but still there are general trends to look out for. The first would be the colour-suppressing gene discussed above. That should, in theory, turn up as white flowers in about one in four of the F2 plants. In the event, I only had one plant with white flowers (plant no.7), but that's enough to show that the inheritance of that trait is working as expected. In this project, the colour-suppressing gene is not what I want, and I will have to select against it in future years as well, as it will be lurking as a hidden recessive in many of the other plants even though they didn't have white flowers.

Of those which showed coloured flowers, I was expecting a majority to have purple bicolour flowers like the F1, as that's controlled by dominant genes and is the trait most likely to express itself. I know very little about the genetics behind pink flowers in peas, if I'm honest, but I was pretty sure it would be a recessive trait and so I had my fingers crossed that it would turn up in a proportion (maybe one in four) of the non-white plants.

And voilà!

It actually turned up in exactly four plants, numbers 3, 6, 10 and 15, which is close to being a Mendelian ratio. These all produced flowers which were a very consistent shade of pink, that is, they were all bicolour pinks with peachy salmon wings and a pale blush pink standard. There was no variation in the expression of the pink colour, other than some differences in how the colour changed over time as the pigment broke down in the fading flowers, where some turned a more dusky, rosy pink than others. But in terms of the essential flower colour, they were identical to one another and identical to the colour of the parent plant, Salmon Flowered, which provided the pink gene. They did all have somewhat larger flowers than Salmon Flowered – flower size was more consistent with that of Alderman – but that's controlled by a different genetic locus.

They were absolutely beautiful and I was thrilled with them.

A second recessive trait which I was expecting to show up in one in four plants was the umbellatum form – which as far as I know is a simple recessive gene which causes the flowers and pods all to bunch together in a cluster on top of a fattened stem. But having never done breeding work with umbellatum-type peas before, I couldn't be sure how this would work out in practice. Again though, it turned out much as predicted. Only two plants, plants 5 and 9, were of the umbellatum type, which is a bit short of a Mendelian ratio but close enough to show the principle of it. There were no intermediate types, all the plants were either umbellatum or non-umbellatum. As far as this project is concerned, the umbellatum trait is not a desirable one and so I'll be having to select against it until all the hidden recessives are eliminated from future generations. The two plants which had this trait were both purple flowered anyway. It may not be a desired trait in this cross but it does look quite spectacular!

All of the pink-flowered plants had what you might call 'normal' form. They produced two large flowers per node, which gave rise to large green pods, much like Alderman. The umbellatum types did tend towards very slightly smaller pods, whereas all the non-umbellatum types had normal size pods, which suggests that smaller pods are a byproduct of the fasciation trait in umbellatum peas. It may simply be that the plant doesn't have enough energy to produce the flowers and pods all together at the same time without compromising on size a little bit. If there was a genetic cause, i.e. a gene in Salmon Flowered which made its pods smaller (as they ARE quite small) then I would expect that trait to segregate randomly through the F2 plants – but it didn't.

Axillary pigmentation is another interesting one. It occurs in all (in my experience) peas with coloured flowers. A purple-pink splodge at the point where leaf meets stem is invariably seen in a plant with purple bicolour flowers. The purple blotch is made from anthocyanin pigment, so it's subject to the action of the same colour-suppressing gene which causes white flowers. A pea with white flowers will have no pigment in the axils at all, because this gene switches off all production of anthocyanin pigment throughout the whole plant. So in a cross between a white-flowered and a coloured-flowered pea, like this one, the presence or absence of colour in the leaf axils is a very early indicator of whether the plant will eventually have white or coloured flowers. The axil pigment usually shows up quite early in the seedling stage, when they've produced their first couple of sets of true leaves.

In the case of this hybrid, the Salmon Flowered parent variety has an unusual kind of axillary pigmentation. It's lighter than the usual type – in fact it's a dusky rose pink, and very soft and subtle. I was thinking it's probably not a coincidence that a variety with unusual pink flowers also has unusual pink axillary pigmentation – there's probably a meaningful correlation between the two. So I was watching my F2 seedlings to see if there was any sign of this correlation, and there was.

The photo shows the subtle dusky pink pigment in the axil of ASF 06, one of the pink-flowered phenotypes. All the other pink-flowered ones had this as well, while the purple-flowered ones had the more normal blotch of purple colour in the leaf axils, and the white-flowered plant had none at all, as expected.

This is bloody useful, actually. If the colour in the leaf axil is a reliable indicator of flower colour, which it does seem to be, subject to errors of interpretation when the shade is a bit ambiguous, then it means you can identify the flower colour a good month or two before they flower.

Right then, so the appearance of pink flowers in roughly a quarter of the F2 plants suggests a fairly straightforward recessive gene at work. I'm sure there is some info out there somewhere about how this works, but the details of which genes control which traits are often buried in papers in academic journals which I don't readily have access to – and even when I do get hold of them, I struggle to make sense of the scientific jargon and it makes my brain hurt. So in my layperson ignorance I'm going to make a speculative guess about what's happening with this pink flower business.

I think that a pink-flowered pea is essentially a purple-flowered pea which has come under the influence of a modifier gene – probably just a single, recessive modifier gene. I think this modifier gene acts on the chemical makeup of the anthocyanin pigment, suppressing the production of blue pigment while leaving red pigment unaffected, so that the flower comes out pink instead of purple.

There are two reasons why I think that. The first is to do with the axillary pigmentation. If there was a gene specifically coding for pink flowers, I can't see why it would affect the colour of the axils as well. But clearly it does, because pink flowers and pink axils go together. Which suggests a modifier gene having a blanket effect on anthocyanin production throughout the whole plant.

The second reason I think this is the case is because of a study which has been done on sweet peas, which are a different genus from edible peas but have a lot in common with them. As I mentioned earlier, the default ancestral colour for sweet peas is a purple bicolour, but some time in the 18th century a mutation occurred which gave us the lovely pink-and-white bicolour known as Painted Lady, which is still widely available today. A study was published in 2017 in the Canadian Journal of Plant Science on the genetic basis of this mutation, and although I don't have access to the paper itself there was enough information in the abstract to tell me what I needed to know: a single base pair mutation means that the flower is lacking the blue pigment known as delphinidin, which is one of the anthocyanins which make up the purple colour in sweet peas. In the absence of delphinidin, the flower becomes pink. In simple terms, if you imagine that the colour of the purple flower is made from layers of translucent blue and pink, removing the blue layer leaves you with just pink.

So that's what I think is probably happening in Pisum sativum as well. If you look at the pink flowers in my F2 plants, they are all bicolours. They are all essentially the same, there's no variation in the colouring. So it seems quite plausible that they are meant to be the default purple bicolours, and that a recessive gene has come along and deleted the production of delphinidin (or whatever blue pigment they're supposed to have) and this salmon-pink bicolour is the result.

As I said, this is just my speculation! I'm sure there are people out there who know more about it than me.

This is turning into a very long post, but I have learned such a lot from growing these sixteen F2 plants!

So let's finish up with a bit about the pods and peas. This project is not seeking to produce edible pods or coloured pods: both parents are green-podded shelling peas, and all the offspring are green-podded shelling peas as well. My aim was to get the kind of big, plump green pods and fat peas found in Alderman, and not so much of the small pods and small peas of Salmon Flowered. In this, the F2 generation has given me what I wanted, because all the non-umbellatum type plants produced pretty good pods and most had good sized peas.

I did some taste tests as well. I was hoping to get as close as possible to the sweet and complex flavour of Alderman and not so much of the pleasant but rather mealy taste of Salmon Flowered. In this I was also very lucky. I tasted three out of the four pink-flowered phenotypes and they all had very good tasting peas, with ASF 06 being the best. Unfortunately ASF 10, which was a lovely plant with beautiful flowers, died prematurely after getting its main stem damaged in a storm. It had only just begun setting pods at that stage and the peas inside were still very immature. I thought I had nothing to lose by leaving the pods on the plant as long as possible in the hope that they would use the residual energy of the plant to carry on maturing a bit. And they did. When I finally harvested the pods, the peas were still quite small but they look like they might, just might, be mature enough to germinate. I didn't eat any of these – I wanted to conserve as many as I possibly could.

Here are the seeds from the four pink-flowered plants after being harvested and dried. As you can see there are a few differences between them. They all have subtle purple speckles on them, except for the salvaged seeds of ASF 10, which were not fully mature. ASF 15 has more of a green colour to its seeds, while ASF 06 and ASF 15 have a mixture of green and tan. The tan seeds are a trait inherited from Salmon Flowered, which also seems to be related, albeit loosely, to the pink flower trait. You may also notice that ASF 06 in particular has some variation between wrinkled seeds and rounded, dimpled seeds. The wrinkled ones are a rule-of-thumb indication of sweetness in peas, because sugar shrinks more than starch does. I probably won't select out the wrinkled ones next year though, I'll grow a bit of both, but I might possibly separate them out into different halves of the seed tray so that I can keep track of whether there's any correlation between wrinkled seeds and sweeter flavour.

That's it now, until next year when I can grow the F3. I would expect all four of the pink-flowered peas to breed true for the pink colour, as it's a recessive trait and they must be homozygous for that trait. It's likely that some of the purple-flowered phenotypes are heterozygous for pink flowers, and will produce a few of them in their offspring – so I will probably grow out some of the best of those to see if I can get some more pinks. But either way, I'm extremely pleased with what has come out of this F2 crop and I'm feeling quite optimistic about the prospect of getting peas with pink blossoms and lovely flavour, within a couple more years.

Please universe, may I have some edible red podded peas?

Golden Sweet x Carruthers' Purple Podded F2. First one to flower.

Today has been a perfect day for hand-pollinating peas. Warm and dry but not too sunny - which creates just the right conditions for pollen to spill its abundance and for stigmatic goo to be in the right mood to receive it. Even more usefully, it was completely breezeless with not so much as a leaf blade stirring. The bitter lessons of trying to hand-pollinate flowers that are thrashing about in the wind, not to mention walking across the garden with a precious blob of pollen on the end of a scalpel, have taught me that pollinations on breezy days are the stuff of futility. All the more so for those of us with waist-length hair, which is guaranteed to flap across your face at the very moment you were trying to deposit a miniscule dab of pollen onto a particularly wobbly and elusive stigma.

As the weather was so perfect for it, I was really hoping to get some useful pollinations done for my red podded pea project. Only trouble is, most of the flowers I want aren't ready. The true red podders aren't even producing buds yet, and my semi-red mangetout line, which I'm hoping to cross with the Luna Trick sugarsnap for some peachy-red snap pods, didn't want to play either. I found a prime pollen-bearing bud on it that was well past the usual stage for self-fertilisation but when I cut it open I found it stubbornly refusing to dehisce.


An F4 plant from my semi-red podded mangetout line, currently struggling under the temporary working name of Peachy, getting ready for some blossom action (but not yet).

So instead I did some pollinations I didn't really need using the flowers I had available. It is worth pointing out that my beloved Luna Trick pea is the result of just such a casual union, in which I used up the last of some Sugar Ann flowers to pollinate a few buds of Golden Sweet just because I was bored. It turned out to be an inspired combination. Today's efforts mostly involved Sugar Snap flowers as females, pollinated with some of my purple F2 plants which may or may not turn out to be any good. I used the opportunity to take some close-up photographs of the pollination process. It's my fourth attempt to take such pictures. Thing is, you really need three hands for it, or an assistant who knows what they're doing. I have neither, so Plan B was to stick the camera on a tripod and use the self-timer for some very cumbersome hands-free photography, which resolved the three-hands issue but gave me some more challenges in trying to get it to focus in the right place when there's negligible depth of field. Anyway, if any of them are any good I'll add them to my previous pea-breeding tutorial.

The hand pollinations are just one aspect of what I'm doing on this crimson seeking project. Since my pièce de résistance in the red podded pea department stubbornly refused to yield me any fibre-free pods, I've been looking at alternative ways of getting them. Which now involves several simultaneous endeavours.

Growing out the rest of the F2 seed from whence the original came. This is the biggest hope. The particular combination of genes I need are a minority class which will only show up in a small proportion of the F2 offspring. I need the yellow pod gene (recessive), two purple pod genes (both dominant) and two fibre-thwarting genes (both recessive). I can't be arsed to look up in a Punnett Square what the actual chances are and calculate the number of plants I need for 95% probability … I'm content to know that it may take a lot of plants in order to deliver the holy grail. This year it's down to luck anyway, as I have only a small amount of F2 seed left and so I can't grow lots of plants. Even the seed I do have is of poor quality because it was grown from a late summer crop (I used to have this trick of growing two consecutive generations in one season to double the speed of my breeding work, but have since stopped doing it because the second crop yields weak seed at best, and at worst yields nothing and just wastes valuable breeding material). So I have about fifteen, maybe twenty plants, and they may or may not offer any red pods. The moment of truth is approaching, as the F2 plants have got flower buds.





They're very pretty buds, with the mauve blush that promises bicolour flowers. Both parent varieties had bicolour flowers, so I'm expecting to see it in all the offspring. The ones shown above are fairly typical and there are others similar, with more or less purple sploshing on the stems and leaves. But I can already categorically rule out any red pods from these two plants or any of the others currently budding. The reason? Green calyx.

Green calyx means green pods. I've noticed from my work with yellow podded peas that there is a direct correlation between the calyx colour and the pod colour. Yellow podded peas are always preceded by pale cream buds which turn into a cream calyx. Sometimes it has green mottling, and pink dapples, but the base colour is always cream. (Have a look at the picture of Peachy above and see how cream the calyx is compared to these F2 buds). I suspect the cream calyx/yellow pod may actually be coded by the same gene. If they are separate genes, they are certainly slapped together pretty tightly on the chromosome, and inherit together. It's not affected by flower colour - you can get white flowers or purple bicolour flowers on a yellow-podded pea, it's just the calyx and pod colour that are inseparable from each other.

When I say that green calyx means green pods, it may also mean purple pods … or partial purple. That's because purple podded peas are in fact green podded. I know that sounds weird, and I've had to explain it so many times on plant breeding forums it's obviously something a lot of people find hard to follow. If you look closely at a purple pod, the very tip where it attaches onto the plant is green. Break it open and it will be green inside. The purple pigment, no matter how intense it looks, is merely on the surface, and the base colour of the pod is green. This is also the reason why all purple podded peas turn green when cooked. The water-soluble anthocyanin pigment is just sitting on the surface and is washed away in hot water.

I admit it did take me a while to work this out. The first time I grew F2 seeds from this cross, it produced red, green, yellow and purple pods in varying proportions. I couldn't understand why a cross between a purple podder and a yellow podder yielded so many offspring with green pods. Neither of the parents appeared to have green pods so where did they come from? The simple answer is that the purple parent is green podded, with the green hidden under the layer of purple. In the great gene reshuffle, some of the offspring end up with green pods without the genes for purple overlay, and so they stay green.

The same principle applies to red pods. They are simply yellow-podded peas with a purple overlay, which combines visually to make deep red. It's not possible to have red pods unless the base colour of the pod is yellow. That's how I know the two buds shown above are not going to give me red pods. A red-podder bud will invariably have a cream calyx, not a green one.

While I shouldn't condone the practice of peeking inside unopened leaf clusters to look at bud colours, I naturally can't resist it. And it's been very encouraging. Because two of the upcoming F2 plants which are not ready to blossom yet are showing cream buds among distinctly yellowy foliage. Even when they're tiny, the cream colour is unmistakable. The cream buds don't necessarily result in red pods, some will just stay yellow, it depends whether the genes for purple overlay are also present. But they do open up the likelihood of it. Also, one of the cream buds is showing a speck of pink colour on the calyx. While I don't have a genetic explanation for it, I have noticed a strong correlation between pink markings on the calyx and red pods. So I'm feeling lucky with this one.


Maybe it's not very clear in this photo, but this developing bud has a cream calyx - ergo yellow pods. Also a tiny pink spot on the calyx which is a good sign.

It's still pot luck whether any of the plants in this small sample will give me exactly what I want, but the solution is in there if only I can grow enough F2 plants. Which brings me on to my convenient back-up plan …


Growing out more F1 plants to make new F2 seed. I still have a number of F1 seeds saved from when I made the original cross. The seeds are good, healthy mature ones too. I started off a batch of about ten F1 plants this year. The beauty of peas and their efficient self-pollination is that you don't need to do anything except grow the F1 plants and save seed from them. They are veritable F2 seed machines. Every pea they produce has a unique individual genome, its own personal reshuffling of all those genetic goodies. Somewhere among the reshuffles is bound to be the specific five-gene combination I'm looking for.


F1 hybrid of Golden Sweet x Carruthers' Purple Podded. Flowering like billy-o and hopefully making lots of nice F2 seeds for me.

What's interesting about these F1 plants is that they are showing massive hybrid vigour, or heterosis. It's a phenomenon brought about by having a mixed up genome, a by-product of heterozygosity. Its cause has always been something of a mystery, though I'm told that some recent research has put it down to an enhanced ability for photosynthesis. Whatever the reason for it, my own observation is that it only happens in certain crosses - though there are degrees of it. And it's for one generation only - you don't tend to see it in F2 plants. This has been especially marked in my current crop, because I sowed the F1 and F2 seeds from the same cross side by side in the same rootrainer tray, and they are now growing side by side out in the garden. And from the moment of germination, the F1 plants rocketed away from their F2 nephews. They grew faster, had thicker stems, established themselves in the outdoors quicker, produced substantially bigger leaves and grew taller. They were also earlier to flower - and still having a burst of surplus energy to get shot of, have thrown out a lot of sideshoots too. Pea sideshoots are usually feeble, spindly things, if they ever get going at all. These are nothing of the sort. They are full-size, chunky, vigorous new branches which look set to flower and make pods.

Something else peculiar about the F1 plants, or one of them at least. I've been writing about the leaf aberrations in my peas this year, which I'm beginning to conclude are probably weather related. I mentioned the fasciation (thickening of the stem) in one of my other hybrids, despite the fact that it's a trait caused by recessive genes. Well, now some spontaneous fasciation has occurred in one of these F1 plants. To my knowledge, there are no fasciation genes in this hybrid, which is not related to the other one - they are completely separate breeding lines. Which leads me to assume that pea fasciation is not solely genetic, and can arise as an environmental reaction. What's even more weird is that the plant in question managed to unfasciate itself by splitting into two stems. A single stem with a well developed sideshoot is one thing, but this is a pair of twin stems growing at the same rate in different directions, equal and opposite.


This F1 pea developed spontaneous fasciation (stem widening) and then split into two separate but equal growing tips. This is not normal for peas!

Edit: I've just done some homework on fasciation, following a link from Rhizowen's wonderful blog, and it seems that spontaneous fasciation from environmental stress is a well known phenomenon - actually more common than genetic fasciation. It's caused by damage to the growing tip by virus, bacteria, insect nibbling or frost - and a reversion back to normal growth is also common. In this case, frost is almost certainly the culprit.

Other avenues. I had just eight seeds left from my original red podded pea. I sowed them, and three fell victim to marauding gastropods. The five survivors are doing well though, and although there will be no edible pods among them, they will enable me to make some crosses. The priority will be to cross them with my two Luna Trick lines, which are genetically similar, being derived from the same original parent variety, but represent a superior form of it with good-flavoured and fully edible yellow pods. Another batch of twenty or so are on the go, thanks to my big haired friend Graham, who grew some last year and gave me back some seeds from the best of them. Again, they won't have edible pods, but they will be priceless for making crosses and might also make the basis for a red-podded shelling pea. Also of course there's the Peachy line which has edible pods which are part-red. That might turn into a variety in its own right, and will certainly be useful for making crosses with the pure red (if it ever gives me any pollen).

The joy of Mendelian segregation ... illustrated!

Nature makes order from randomness.

The photo above shows one of the pods from my Yellow Sugarsnap Project with peas segregating for seed colour. The pod is from one of my F2 hybrid plants (the second generation after the original cross) so the peas inside are F3. As immaculate as this alternating pattern is, it's entirely random.

I've just spent three days typing up descriptions of all my little packets of F3 seed from the Yellow Sugarsnap Project into a nice tidy table, and even as I handled each of my sixty-two seed packets (each plant's seeds carefully saved separately) and stared at them hour after hour I didn't notice the pattern. I noticed that some of the packets of seed are very uniform while others show a bit of variability. I thought that factor might be significant, so for each one I wrote down how variable the seeds were, and which traits they varied for. Sometimes it was size or colour, but more often it was a case of a few wrinkly seeds showing up in a batch of smooth ones. I dutifully jotted all this down but I still didn't notice the pattern. D'uh!

And then I was asked to do a little recorded talk about Mendel and his peas for a University of Bath podcast, just a very brief grounding in the history of genetics for psychology undergraduates. Not trusting myself to not screw it up, I did some refresher-research on Mendel. And in doing so I thought very hard about his experiments, and how he'd been the first person to notice the recurrence of 3:1 ratios in inherited traits. And it was only then that I twigged that there was a pattern in the seeds I'd collected from my pea project. So I raked them all out of the box and sorted them into different groups, and ker-ching! There it was. A beautiful and very obvious ratio.


As like as two peas in a pod? These F3 seeds from my Yellow Sugarsnap Project vary from smooth to wrinkled in the same pod, as well as varying for colour.

As a romantic idle speculation, I wonder whether Mendel found the same thing in his peas and got the initial idea for dominant/recessive segregation from it. Peas have this wonderful advantage over pretty much all other vegetables, that certain traits show up visibly in the seeds. If Mendel had been experimenting with tomatoes or brassicas this wouldn't happen because the seeds all look very similar no matter how different their genes are. He would have to actually grow the plants to see the differences between them. But with peas being the way they are, he must have seen a pattern very similar to what I have here.

The pattern is this: a number of my seed packets from the F2 plants have perfectly uniform round peas, with no wrinkles. A similar number have all wrinkled peas, with not a single round one among 'em. But a larger number have got variability for wrinkliness. And in every one of these cases they have, roughly speaking, a quarter wrinkled and three-quarters round. There are no other ratios. None of the packets have mostly wrinkled with just a few round, or even half and half. They all have an approximate 3:1 ratio in favour of round peas. A Mendelian ratio in other words. In fact there are two Mendelian ratios at the same time. The packets of round or predominantly round seed outnumber the packets of wrinkled seed by about 3:1, while the ratio of round to wrinkled within each of the variable seed packets is also 3:1.

I sorted the seed packets into types. On the left are all the seeds which are completely round with no wrinklies. On the right are the ones with all wrinklies and no roundies. In the middle are the packets which show a mixture of types. There are roughly twice as many in this middle group, as you can see.

Wrinkliness is one of the traits Mendel experimented with, and he found it to be recessive to roundness. This is now known as the R locus. The round-seeded allele is R and its wrinkle-seeded alternative is r. My original cross was between Golden Sweet (RR) and Sugar Ann (rr), so the resulting F1 hybrid must have had a genotype of Rr. Recombining those Rr genotypes in the F2 generation can go any of four ways, with visible effects in the seeds, like this:

Genotypes in the F2 plants can clearly be assigned to their four respective groups.

Why does seed wrinkliness matter? Well, it's a very useful trait for pea breeders to look out for because it's a rule-of-thumb indicator of sweetness. Sugars shrink more than starches do within pea seeds, so the sweeter ones tend to end up more wrinkly. A high sugar content doesn't guarantee a good flavour (as I found in my taste tests with these) but it helps.

It's obviously very useful to be able to identify the seeds which are likely to produce plants with sweet-tasting peas before you've sown them. If I want to breed a sweet-tasting variety I can just pick out and sow the wrinkly seeds and not the round ones, which will greatly increase my chance of getting what I want. This is a really unusual situation, and only works because the desirable trait shows up in the seed itself in an obvious way, when most other traits don't – you have to grow the plants to find out what their genetic make-up is, and even then you can't always tell. It's only because wrinkliness is recessive that I can be confident it will breed true.

Let me explain from a practical point of view. Dominant traits are a pain in the backside for plant breeders to work with. Say I wanted to breed a new pea with purple flowers, based on a cross between a purple-flowered and a white-flowered variety. Purple flowers show straightforward dominance in peas, so I would get ALL purples in the F1 generation followed by an F2 generation which was three-quarters purple and a quarter white. So I would obviously proceed by saving seed from all the purple-flowered F2s and removing the whites. When I sow the seeds from the purple-flowered plants, will they simply produce more purple-flowered plants? No, only a third of them will be true-breeding for purple. The rest will still have the recessive white-flower allele lurking in their DNA, hidden by its dominant purple twin. Although they look like true purples on the outside, those plants will again produce a 3:1 ratio of purples to whites. Unfortunately there's no way to tell which are true-breeding and which aren't, other than by growing them and removing all the whites in each generation until they eventually stop showing up.

Recessive traits, by contrast, are a joy. They show up in smaller proportions of course, but once you have a plant with the requisite pair of recessive alleles it should breed true from then on without any further mucking about.

That's why the sweet-wrinkly seeds showing up in a Mendelian ratio is such a godsend. Laying all these peas out on my desk in their individual packets, I can see their exact genotype for the R gene at a glance. The round seeded ones are RR and will breed true for roundness. The wrinkled ones are rr and will breed true for wrinkliness. The ones that are mostly round with a few wrinklies are Rr or rR (which amount to the same thing) and will continue to show variability in their offspring.

This is incredibly handy. Not only can I identify the sweet ones without having to grow them all and taste them, I can see which of them are true-breeding for sweetness/wrinkliness. If I want to be sure of getting a full complement of wrinkliness in my plants for ever after, I can instantly pick out the ones with the fully recessive genotype and Bob will be my uncle.

The reason this is possible is because this segregation for seed type is showing up within different peas on the same plant. Compare that to the situation with flowers. If some of the plants were obliging enough to produce a load of purple flowers and a smattering of whites all on the same plant, that would be great. I would know those were not true-breeding for purple. But they don't. They produce all purple flowers and keep the whites hidden in their genome to pass on to their offspring unseen.

OK, so we've established that the plants which produce only smooth, rounded seeds must be RR, and because they have a matching pair of alleles their offspring will also be RR. The technical name for this is homozygous. Exactly the same is true of the plants which produced only wrinkled seed. They are also homozygous, because their genotype must be rr and so all their offspring will be rr too.

The plants which produced a mixture of round and wrinkled types have to be heterozygous. Instead of a matched pair of alleles they have one of each type. That means that when they make seeds they will randomly pass on the four possible combinations to their offspring: RR or rr (which are both homozygous and will breed true) or Rr or rR (which are heterozygous and won't). The heterozygous seeds will express their dominant allele and hide their recessive one, so they will look the same as the RR seeds, and so once again there will appear to be a ratio of 3 rounded to 1 wrinkly.


Note that it's the plants which produced these seeds which are heterozygous, not the seeds themselves. Half the seeds in the heterozygous batch will actually be homozygous, but the other half remain heterozygous and will produce variable offspring which are half homozygous and half heterozygous, and so on ...
These seed packets are all siblings from the Yellow Sugarsnap project ... I still can't get over the amazing diversity made by this one simple cross!

With the two quarters of homozygous seeds separating out like this, you can see that in each generation half the heterozygosity is lost. If continued for a few generations it will all but disappear. That's how new varieties are stabilised.

In practical terms, what does that mean for these packets of variable seeds from the heterozygous F2 plants? Well, I know that I have all four classes mixed up here in approximately equal amounts, and I can see which seeds are homozygous (true-breeding) for wrinkliness, because they're wrinkled. Unfortunately I can't see which ones are homozygous for round seeds, because they look exactly the same as the heterozygous ones. Hence this 3:1 ratio of round to wrinkled. If I were to sow all these seeds, I would find the same 3:1 ratio in the next generation too, and onward.

Finally, a little reminder that all I'm looking at here is the R locus, the gene controlling wrinkliness. That's just one of many thousands of genes in every pea. Segregation is taking place at every other locus at the same time! If I select identical-looking wrinkled peas, I can assume they will be true-breeding for wrinkliness but they may differ enormously in other traits.

Wow, my head feels weird now.

The joy of genes ... illustrated!

Patient readers who have put up with me banging on about gene segregation and F2 hybrids ... here's a little photo sequence from one of my breeding projects to show the process in action. I hope this will be a lot more interesting and meaningful than my simply talking about it, since it shows what amazing and beautiful diversity is locked up within every seed. If it inspires you to have a go at some hybridisation yourself ... so much the better.

OK, so these are pictures of pea seeds from my Yellow Sugarsnap project. It matters not what the objective of the project is or how close I am to achieving it ... this is just an illustration of what happens when you cross two varieties.

In this case I started off with Golden Sweet, an old heirloom supplied by the Real Seed Catalogue, and Sugar Ann, a bog-standard commercial variety from a garden centre.

The original parent varieties. Golden Sweet (left) has dimpled tan or grey seeds with purple speckles, while Sugar Ann has pale grey-green or cream seeds which are more wrinkled and slightly bullet-shaped.

So I made a cross between these two varieties, thus creating an F1 hybrid, and this is what the seeds looked like:

F1 hybrid between Golden Sweet and Sugar Ann

Sorry this is a bit of a small sample, but I'd already planted most of my F1 seeds by the time I took the photo. Anyway, you may notice that the F1 hybrid seed looks exactly the same as the original Golden Sweet seed. There's a good reason for that. The embryo hidden deep within the seed has the hybrid DNA made by the cross-pollination, but the rest of the seed (including its outward shape and colour) is the product of the mother plant. Therefore it looks just like any other seed produced by the mother plant. If I'd done the cross the other way and used Sugar Ann as the mother plant, then all the F1 seeds would have looked like Sugar Ann.

The next step was to grow the F1 seeds and collect seed from them, giving me the F2 generation. I didn't make any further crosses ... as peas are self-pollinating, all I had to do to obtain the F2 seed was to grow the F1 plants and allow them to produce seed naturally. This is the result:

F2 hybrid between Golden Sweet and Sugar Ann (i.e. the seeds from the F1 plants)

Hey up, now we've got something happening. The F2 seeds no longer look exactly like the Golden Sweet parent. In fact if you look closely they're all different. The differences are quite subtle but they vary in colour, size and shape. Some are wrinkly while others are smooth or dimpled. Some have purple speckles, others are plain. They show a jumbled up mixture of traits from the original parent varieties, caused by the random segregation of genes from both parents.

This is the point where plant breeding becomes immensely fun. Because every one of these F2 seeds produces a plant that is unique. And once again I don't need to do any crosses, I just grow the F2 plants and let them set seed naturally to produce the F3 seeds. And I get THIS:

F3 hybrid between Golden Sweet and Sugar Ann (i.e. the seeds from the F2 plants)

This is actually just a random sample, the first nine plants to reach maturity. There were many many more variations, but these few are enough to show you what's happening. I've saved seed from each F2 plant individually, and you can see that there is some consistency in the seed type for each plant, but HUGE variability between plants. Plant 58 produced seeds the same shape as Sugar Ann but a much brighter green and with purple speckles. Plant 02 produced seeds the same shape as Golden Sweet but green instead of tan. Plant 25 produced exceptionally wrinkled seed with no speckles. Plant 09 produced large round smooth yellow seeds which are totally unlike either of the original parents. Plant 14 shows some variability within itself but again a spectacular diversion from the original parent varieties, because the whole seed coat is sploshed with solid purple with a few bright greens and pinks thrown in.

Same image, detail

Every one of these packets of F3 seed is a brand new, unique variety in its own right. I could give them all names and launch them on the world. There wouldn't be much point doing so, partly because their offspring would still show some variability and further segregation (so they need to be stabilised for a few more generations first) but also because they won't all be worth pursuing. At a glance I'd say that Plant 09 with its big smooth yellow seeds is probably not going to taste good. In fact I did eat some of its seeds while they were still fresh and they were hard, mealy and bitter. By contrast, the exceptionally wrinkled seeds of Plant 25 indicate an exceptional sweetness, confirmed by taste tests, and that one is probably worth pursuing. Plant 37 also looks useful, as it has the supersweet ultra-wrinkled seed combined with pretty purple, pink and green colouring. There's enough interesting material here to keep me occupied for years. All from a single cross!

Anyway, what I hope this illustrates is that all these seeds are different from the original parent varieties in ways I couldn't have imagined when I made the cross. There are some familiar traits showing up, but also a lot of brand new ones which weren't displayed by either parent. And some of those brand new traits are really quite exciting.

What these pictures show is segregation for seed-coat colour and seed shape. Because in peas those two traits are readily observable. Of course the same level of segregation is happening to ALL traits right across the genome, with potentially millions of different combinations. I hope this gives some idea of how much diversity and scope for new varieties is possible just from making one simple cross-pollination.

Red-podded pea update

All together now ... oooooooooooh!!

The red-podded peas have now completed their life cycle, and in fact have started a new one, because I've already sown a batch of their F3 offspring.

Red-podded peas were an unexpected gift from my Purple Mangetout Project, which I'm doing on behalf of The Real Seed Catalogue. I really have to thank Ben of Real Seeds for making it happen. He sent me the Golden Sweet seeds to experiment with and I don't think I would have thought of crossing Golden Sweet with a purple podded pea if he hadn't suggested it. It was a good choice ... I think there's a distinct lack of genetic diversity in peas generally, and as I'm discovering in this and other projects, Golden Sweet is something really different and brings a heck of a lot of useful genetic material into the mix.

Among my F2 plants there is just one, known as GSC15, which has pure deep red pods. GSC stands for "Golden Sweet x Carruthers", followed by a number. It's just my own little code for identifying individual plants.

Here's what GSC15 looked like at various stages.

Ooooooh ....

Aaaaaah ...

Yeah ...

This plant always looked special and highly coloured, and in fact I posted a couple of photographs of it when it was just a seedling, because it was the most beautiful of the seedlings. It's turned into an amazingly beautiful mature plant. It has a whole range of candy colours as well as the spectacular crimson pods, even producing two-tone pink and yellow tendrils. It's an improvement on Golden Sweet, the parent it inherited its colours from. It grew vigorously and elegantly, and also had the grace to produce two pods per node. It really is a winner. There has to be something imperfect about it though, and I think that might be in the pods themselves. It's clearly not a true mangetout because it has some fibre and leatheriness in the pods. But I'm not sure it's cut out to be a sheller either ... the one pea I ate was a little bit starchy. It would be a shame if you couldn't eat the red pods anyway, so I'm looking to develop a fibre-free version.

This is not really a problem. The next generation is likely to show more segregation for hidden recessive traits and there's a good chance, I think, that I'll get something with the same strong colour but a true mangetout, or sweeter peas.

GSC15: the young leaves have a yellow tint to them, and a deep pink blotch in every leaf axil. The flowers are bicolour maroon and mauve, a trait inherited from both parents.

Even the tendrils are beautiful. The stems are peachy pink and the tendrils bright yellow. It's a difficult thing to photograph and this really doesn't do justice to it but you can see the contrast here as it clings to the ordinary green tendril of one of its siblings.

Yellow-podded peas always show a lot of yellow in the young leaves as the plant reaches the flowering stage, and of course red-podded peas ARE yellow-podded, with an extra layer of purple over the top to create the red. So GSC15 shows all the colours you'd expect in a yellow pea, with some additional pinky colours. The calyx (pixie hat) on the flower buds are pale cream but show quite a lot of pink streaking, which is something I haven't seen in any other plants ... but it is a natural enough combination of parental traits ... Golden Sweet is the source of the cream calyx while Carruthers' Purple Podded provides the pink streaks.

GSC15 plant top ... yellow leaves with a very bold, bright pink splodge in the leaf axil and also some strong pink markings on the flower buds.

Although GSC15 is the clear champion of this batch of plants, there were other red-podded peas which were mostly red but maintained some of the underlying yellow. Some more than others. GSC09 is quite promising ... it's a true mangetout, and the one pod I tasted was very nice. The red colour is quite patchy in places though, with only a few showing a dominance of red. The photo at the top of this post shows a GSC09 pod, one of the less yellowy patchy ones. Again, I'm hoping some further segregation in the F3 generation will give me more mangetouts like this but with a stronger colour.

Initially the red podders were all from my Golden Sweet x Carruthers cross, but later a couple showed up in the Golden Sweet x Desiree cross as well. The GSD plants were both dwarf phenotypes with mostly yellow pods and only a spraying of red. They don't look as promising as the GSC ones but it's interesting that this happened at all ... it suggests that any cross between a purple and a yellow podded pea could potentially produce red pods, and it's not an exclusive feature of the Golden Sweet x Carruthers cross.

Flowers on a red-podded Golden Sweet x Desiree hybrid. Again it's a beautiful highly coloured plant with two flowers per node, although it only grows to a foot or two in height.

Depending on what traits show up in future generations, I may develop the semi-red peas into a new variety in their own right. They have a charm of their own, and although they're not as spectacular as the true reds, they are still a unique colour break. Some of them are yellow with just a bit of red edging, others are lightly "sprayed" with red all over, giving a peachy effect.

Bicolour yellow and red pods, one of the Golden Sweet x Desiree plants.

The next stage is to grow the seeds from all the red-podders. Normally in a breeding project you have to cross the F2 plants with each other to produce the F3. As peas are self-pollinating, however, all you have to do is leave them to produce seed naturally. There's still time for me to get another generation grown in 2008 as long as I start them off pretty promptly, so I've been harvesting the pods as soon as they reach maturity and drying them in trays indoors. I judge "maturity" as the time when the calyx starts to dry out and the top of the pod (where it joins on to the plant) starts to look a bit sunken and leathery. I save the seeds from each plant separately, which is a lot of work but enables me to keep track of everything in future generations, and learn from it.

In addition to starting off a batch of F3s, I've sown another lot of F2s as I still had plenty of F2 seeds left. I'm hoping there will be some more red-podders among them which will give me more material to work with and hopefully more genetic combinations.

The first few GSC15 pods drying indoors. The pods go purple as they dry out, and the peas stay green but shrivel to a tiny size. YSS10 is one of the beautiful bicolour-flowered types from my yellow sugarsnap project. I alternate them so they don't get muddled up.

Yellow Sugarsnap Project: all podded out

Outtakes from my yellow sugarsnap pea breeding project.

Now that the yellow sugarsnap project F2 generation has reached full maturity I'm having to decide which lines I want to keep seed from. 63 plants to choose from, each one unique. I could just keep seed from all of them, but that's a lot of work and cataloguing for seeds I will probably never use. So I'm just keeping the most interesting looking phenotypes. The yellow sugarsnap, obviously, and the ones with pink and white flowers. And a few others which had really good flavours or other interesting traits.

I have to be careful though, because they will all have hidden recessives which might turn out to be useful in future generations. When I choose not to save seed from an F2 plant, its unique genotype is gone forever. It's a bit daunting to make these choices because it feels like I'm playing God, or playing Darwin at least. But it's gotta be done, and the inevitable manifestation of evolution is that the weakest get eaten.

Yummy.

Eating F2 hybrid vegetables is a curious experience. F2 plants show a lot of diversity, and that includes flavour, texture and cooking qualities. They look weird on the plate: big ones and small ones, greyish or bright green or dark green, compactly wrinkled or smooth and fat. If you're used to homogenised supermarket peas it's really difficult to imagine what it's like to have a mouthful of diverse peas. It's a sensory overload, like your brain doesn't know which sensation to register first.

Green and yellow sugarsnaps side by side

A yellow mangetout with bulging peas and curly pods (this seems to happen when the pod wall is porcelain-thin)

I did taste tests on as many of the plants as possible and collected the data. The first plant to flower, whose photo I triumphantly posted a few weeks ago, was beautiful to look at but when its peas matured they were hard, mealy, starchy and bitter. By contrast there were some sugarsnaps with ambrosial sweetness.

There was a lot of diversity in the pods too. As the top photo shows, I picked and ate a lot of the green mangetouts, along with some green sugarsnaps and yellow mangetouts, as they were surplus to requirements. Most of them were pleasantly enjoyable, except for one of the yellow mangetouts which turned out to have an inedible inner lining of fibre. So inedible, in fact, it required to be spat out on the side of the plate. I hadn't been expecting that. Diversity is one thing, but both the original parent varieties were mangetout types, so how does an inedible-podded variant suddenly show up in the cross? Well it's a longish story.

Normal peas have a fibrous membrane on the inner surface of the pod, and it is not a nice thing to chew on. What makes a mangetout a mangetout is that they don't have this membrane. Or if they do, it's thin and soft enough to be edible while the pod is young. The mangetout trait is recessive, and just to make it complicated it's controlled by two different genes which function independently.

On an individual basis, gene p on chromosome 6 and gene v on chromosome 4 both seem to fulfil pretty much the same function: reducing the fibre content, to produce mangetouts which are edible-podded when young but may get fibrous later. When you get both these genes together they combine forces in a harmonious way, and fully edible fibreless pods result.

When I selected the mangetout varieties to use as parents, I had no idea which of these two genes were present and in what combination. A mangetout variety could be ppVV (expressing the recessive p gene) or PPvv (expressing the recessive v gene). Or it might have the ideal mangetout genotype ppvv (expressing both recessive genes).

IF one parent is ppVV and the other PPvv, crossing those two varieties will produce offspring with fibrous inedible pods. That's because they create F1 offspring which is PpVv. The two recessive mangetout genes are still there, but overshadowed by two dominant non-mangetout genes. Only in the next (F2) generation can the mangetout trait reassert itself, and only as part of a diverse mixture of mangetouts, semi-mangetouts and inedible fibrous pods.

Or, if you want to look at it in terms of possible gene combinations:
ppVV + P_vv + P_V_ + ppv_

To be honest I didn't think to check whether my F1 plants were mangetouts or not. I saved all of them for seed and didn't notice what sort of pods they had. In the F2 generation though, I have a large number of inedible pods. Especially among the sugarsnap types. If I'd done my homework properly I should have anticipated this, but I didn't. So I have a lot of sweet juicy pods and sweet juicy peas separated by a layer of chewy gristle.

And the bad news ... this includes my coveted and one and only yellow sugarsnap. It has a gristly fibre layer. Can I just say: arse!

All is not lost though. Depending on its exact genotype, I may be able to get some pure yellow sugarsnaps from its offspring. If the recessive mangetout genes are still in there, they will express themselves next time round. I sowed the seeds this morning and will have to wait and see. If not though, I will have to grow out another crop of F2 seeds in the hope of finding another yellow sugarsnap with a more promising genotype.

Ah well, it's all part of the fun. I can't be getting instant exciting results like the red-podder on every project.

Beautiful sunlit colours and a mix of yellow and green pods

Of the six plants which had beautiful pinky-white flowers, five are gristly sugarsnaps but one has perfect edible pods. I will save and sow seeds from all of them, for the reasons outlined above ... some will most likely produce fibre-free offspring. Meanwhile they're showing some beautiful colours as the seeds dry out, as they've all inherited a gene from Golden Sweet which produces dark purple speckles on the seed coat.

Pea update: purple mangetouts

F2 plants from a cross of Golden Sweet x Carruthers' Purple Podded. The colours are stunning when the sun shines on them.

Pictures speak louder than words with these, I think. These are for the Real Seeds purple mangetout project, F2 seedlings from two crosses: Golden Sweet x Desiree and Golden Sweet x Carruthers' Purple Podded. Hopefully a majority of them will turn out purple podded, and from those I hope to find some which have the recessive mangetout pod type. Mangetout peas lack the gristly layer of fibre on the inside of the pod.

All the parent varieties used in this project have purple colouring in one form or another, so it's not surprising that all the F2 plants are showing purple colour. Theoretically they should all be genotype AADD. It is variable though. Some plants have a dark purple axil splodge, some have red edges or red flushes on the leaves, some have rosy stems and tendrils.

They're all beautiful.

Golden Sweet x Carruthers' Purple Podded with non-serrated red leaf margins, pink stems and green tendrils

Deep rose blush on stems and tendrils on a Golden Sweet x Carruthers' Purple Podded plant, and even on the back of the leaves. This one has serrated edges.

I suspect some of the strong purple colouring on these plants has been brought on by the intensely cold nights we've been having. They've been snowed on and endured two or three hard frosts. Tough little things.

Some of the plants have serrated leaf margins, similar to the yellow sugarsnap project. They also share the tendency for dwarf plants to have darker leaf colour ...

Pink stems on a Golden Sweet x Desiree plant. This one is going to have a dwarf habit, it's very low and bushy and you can see the slugs have already been at it. Notice the dark green leaves compared to the yellowy green of the one in the picture below.

A big dark smudgy axil ring on a Golden Sweet x Desiree plant, which also has some purple spots on its leaves.