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Thursday, 22 May 2014

Epistasis and the D gene

I don't know the exact details of the genetics of all my pea plants. Many of the experimental varieties have their own pages on the website of the organisation that I got them from, which lists the genes the researchers know they have, but many others do not. I'm attempting to fill in the gaps by observing all the plants myself.

With this aim in mind, I went out with a notepad and assessed my plants with respect to the D gene. This controls whether pigment is deposited in the axils.

This is Purple Podded. And here's a little plant anatomy - the two leaf structures attached directly to the stem are called stipules, and that dark pink bit is the axil.

To be fair, the axil colour isn't really relevant or at all since I'm mostly interested in pod and flower colour, but it's going to illustrate an important genetic concept.

Purple Podded there has a single axil ring, caused by an allele of D called Dco. Pretty, isn't it?

And Golden Sweet has a double ring due to its Dw allele, which I think is even more attractive.

Sometimes you can get information about other genes from the axil pigmentation, for example in Salmon Flowered, which has only a little pigment. I suspect this is the result of something like the b allele modifying the anthocyanins from a deep red-purple to a paler pink, but I don't know yet whether Salmon Flowered carries b or something else entirely.

Finally these baby Telephone plants don't have any axil pigmentation at all, because of the d allele (no axil pigmentation), right? Right?

Well, not necessarily.

There's another gene that could be at work here - a. The dominant allele, A, makes the plant produce anthocyanins. The recessive a means the plant can't produce any anthocyanins at all, ever, no matter what other genes it has for where to put them.

Imagine it like a factory line building the plant. Department A is responsible for turning on the anthocyanin-making machine, and Department D is responsible for delivering that pigment it to the axil specifically. Other departments deal with delivering it to the flowers, and some to the pods (i.e. the Pu and Pur alleles I talked about in the last post). The B Department along with some others control the exact recipe that makes the pigment, changing its colour. Some will distribute pigment in a specific pattern.

None of these other alleles can do their job if the anthocyanin machine isn't switched on in the first place, because there's no anthocyanin to work with at all.

The effects of these genes are essentially hidden by a, and this is called epistasis - a is epistatic to all those other genes that do something with anthocyanin.

So my Telephone pea might have no axil pigment because of d, or no anthocyanin at all because of a. I can't tell by looking at them right now, but I know it has white flowers and green pods so a is likely.

The easiest way to tell would be to make a cross with a pea I absolutely definitely know is aa but also has anthocyanin-depositing alleles like Pu and Pur (purple pods) and Dco or Dw (axil colour) and the ones that allow flower colour. If Telephone has aa the offspring will all also be anthocyanin-less. If it has A but all the other genes are the recessive non-delivering types, we'll see pigment in the offspring. I don't have the necessary breeding material to do this but to be fair, right now it doesn't matter much to me anyway.

And it might all become clear as a result of other crosses I'm planning to make.

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