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Wednesday, 21 May 2014

More about red pods and crosses

Although I started late, my peas are doing well. The tall varieties are about 2ft tall (of course the short ones are shorter!) now and they all look absolutely beautiful, and it probably won't be too long before they think about flowering, and then I can start to actually make crosses.

The first one I'm going to talk about is Golden Sweet x Purple Podded. I've mentioned this before, but in order to get red pods I need yellow pods with purple/red anthocyanin pigments laid over it - Golden Sweet has the yellow, and Purple Podded of course has the purple.

There are three genes involved here. The first is gp - the Gp allele makes green pods and is dominant over gp, which makes yellow pods. Golden Sweet has gpgp, and Purple Podded (remember that the pods are really green, with added anthocyanins) has GpGp.

If we make this cross, the resulting progeny (which we call the F1, or first filial) will all inherit a gp from their yellow parent and a Gp from the green one - they'll be Gpgp. Because Gp's dominant over gp, the plants will have green pods, and they'll all be identical.

In the second generation though, things get interesting. Breeding F1 with F1 lets the alleles combine in a way that we can see variation in the F2.

Now peas are helpful here, because they are usually completely self-pollinating - all I have to do is grow the F1 and they'll produce the F2s with no interference from me at all. And this has another benefit, which I'll explain later.

So when F1 breeds with F1, we get Gpgp x Gpgp. Each plant will get an allele from each parent, and what alleles they get are randomly assigned. Some will get both Gp from the "mother" and Gp from the "father". Some will get Gp and gp. Some will get gp and Gp. And finally, some will get gp and gp. All four possibilities are equally likely so we'd expect that 3/4 of the offspring would show green pods (those that are GpGp, Gpgp and gpGp (which is identical to Gpgp and so usually lumped in with it)) while only 1/4 show yellow pods (gpgp).

I don't want any Gp at all. And the beautiful thing about it is, if I only save seed from the yellow-podded plants I can be absolutely sure that I've removed it entirely from my gene pool - I know that none of the yellow-podded plants have Gp because if they did, they'd have green pods! So that's step one accomplished. Easy.

Pu and Pur
Of course as well as the yellow base colour to the pod, I also need those anthocyanins to get red. This is controlled by two genes, Pu and Pur. We want both of the dominant alleles Pu and Pur to get the full pigmented effect - pupu purpur has no anthocyanin and will have either green or yellow pods depending on what combination of Gp/gp it has.

We'd expect 3/4 of the plants will have at least 1 of the Pu alleles necessary, and 3/4 to have at least one Pur. This means that we'd expect 9/16 to have at least one of each and therefore have full anthocyanin (because of maths which I won't go into here).

So we've eliminated all the non-yellows right off. But the anthocyanin is trickier - a good amount of the correctly-coloured plants will be secretly carrying at least one recessive allele that we don't want, and we can't tell right away whether they do or not. These "mixed allele" plants (the technical name for an organism carrying two different alleles for a gene is a heterozygote, while one with identical alleles is a homozygote) will end up combining recessive genes in its offspring, which will give us yellow pods. Bad.

So how do we deal with this, and get rid of those pesky hidden recessives? One way is to keep every single F2 plant separate and keep notes on which ones throw yellow-podded offspring in the F3. Then we can go back and grow a new F3 which only contains the lines that didn't have any yellow at all, and can then be fairly certain that we've eliminated all the recessives.

But peas are friendly plants - they grow close together and support each other and get all tangled up with each other and are extremely difficult to keep isolated. that seems like far too much work for me, so I'm going to exploit the fact that peas self-pollinate.

When a plant self-pollinates, it is both the father and the mother of the new seeds it produces, and no other plant is involved. The PuPu plant which makes up a segment of our population can only produce more PuPu plants, and that's fine with us, that's what we want. No need to worry about them at all. There are no pupu plants because they were yellow-podded so we didn't save their seed. The Pupu plants will behave like our original F1s, and produce about 1/4 PuPu, 2/4 Pupu and 1/4 pupu (yellows which we'll eliminate again). You can see that this will give us overall more homozygotes in the population and reduce the number of the recessive alleles, and if we continue to do this we'll get fewer and fewer heterogygotes each year without me doing anything at all apart from not bothering to collect seeds from yellow pods. In 7 years (or less if I'm lucky) things should be stable and it'll be extremely rare to see any yellow pods at all. Exactly the same applies to Pur and its selection will happen simultaneously.

Hopefully somewhere along the line I'll get red-podded peas that taste nice, too!

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