Hybrdizer Academy 101: question about recapturing 2 recessives

[katwest]

I'm trying to recapture 2 recessives in one seedling: one recessive is color and one recessive is shape. How many F2 seedlings do I need to grow out to recapture the 2 recessives in one plant?

When I do my Punnet calculations I get 1 seedling out of 16 has the 2 recessives.

I need to figure out my probabilities next unless someone already has an answer....

Oh, and here's a picture of a Beauty King x Mystery Cross F2 seedling from Brad Gates that I grew out this year.

[katwest]

Oops, first I need spelling academy 101, hybridizer.

[KarenO]

odds are just mathematic probabilities. Could be one, could be a thousand in real life.
genetic roulette and that's the fun part
If the recessive traits are expressed they are fixed in the phenotype of all future uncrossed offspring.
Trouble is often more than one gene is at work in such characteristics as shape and color so it is difficult to predict ahead of time, especially if you do not know the exact pedigree of the parent plants. and then of course there is taste, maturity, production, disease susecptability.... honestly its a crapshoot stabilizing an OP tomato probabilities notwithstanding.
Bottom line is grow out as many as you have room for and hope that among them is something worth working on. Keep an open mind as you might find something that is actually better than what you are trying for... If not, grow them out again for round two of f2 selection or back to the drawing board and make a new cross.
Don't let this or anyone else discourage you. It's fun! good luck!
KarenO

[AKmark]

I saved this awhile back because of my many projects.
http://www.tomatoville.com/showthread.php?t=38505

[katwest]

Super helpful link on probabilities! I think my answer is below:

Number of plants for a certain probability in recovering 2 recessives:
80%: n ~ 24 plants
95%: n ~ 46 plants
99%: n]~ 71 plants

[Fred Hempel]

There is a 1/16 chance that you will get both traits homozygous. However, you don't need to grow out a large number of plants to make sure you hit the 1/16 lottery.

If you are working with F2 plants from an F1 parent that was heterozygous for both traits, one in four plants will be homozygous for either trait. Which means you have a greater than 1/4 chance to get a homozygous recessive for one or the other traits. You can pick any plant that is double homozygous (the recessive trait is evident). Even if you don't see the other recessive trait, there is a 2/3 chance that the other recessive trait is present as well (in a heterozygous arrangement).

So, you can grow 8-12 plants, look for double recessives for either trait and collect all of the single double recessives with a very good chance that the second trait will show up phenotypically in 1/4 plants in the next generation.

[katwest]

Thanks Fred. Very helpful and interesting. How many F3 plants would you grow out in using your two-step strategy (does it have a name)?

I see the obvious benefit in terms of space, but you lose a year of progress so I suppose it depends whether space or time is the priority. I have a really small urban farm so space is always an issue for me - but I'm also impatient to see results.

[Fred Hempel]

You don't really lose a year of progress, because you need to go approximately 7 generations to get stable, and it doesn't really matter which generations your two recessive genes of interest get "fixed". You still have to continue to get all of the other genes homozygous.

What I described to you is my general method of looking through F2 and F3 populations. I think perhaps too much interest is often placed on getting the exact F2 you want, and too little effort is made to continue to select at the F3 generation.

There is still alot of variability at F3, and if you have saved seed from a few F2 families that are most exciting to you, the F3 can be a gold mine of new variations.

Even though I am growing in a field, and I can grow thousands of plants, I very rarely grow out more than 20 plants in the F2 generation, and my rule of thumb for my most exciting projects is usually 8-10.

I would say most often am growing out only 5 F3, but if I am looking for something that might be recessive, I might grow out 10 F3 plants (for each F2 plant that might be heterozygous for something).

Originally Posted by katwest

Thanks Fred. Very helpful and interesting. How many F3 plants would you grow out in using your two-step strategy (does it have a name)?

I see the obvious benefit in terms of space, but you lose a year of progress so I suppose it depends whether space or time is the priority. I have a really small urban farm so space is always an issue for me - but I'm also impatient to see results.

[katwest]

Thanks again Fred! Using your method would certainly allow me to dabble in more crosses at the same time with all the space saved.

Any other sage advice?

[Fred Hempel]

Don't sacrifice flavor for novelty.

[katwest]

LOL, never!

[Darren Abbey]

Originally Posted by Fred Hempel

There is a 1/16 chance that you will get both traits homozygous. However, you don't need to grow out a large number of plants to make sure you hit the 1/16 lottery.

If you are working with F2 plants from an F1 parent that was heterozygous for both traits, one in four plants will be homozygous for either trait. Which means you have a greater than 1/4 chance to get a homozygous recessive for one or the other traits. You can pick any plant that is double homozygous (the recessive trait is evident). Even if you don't see the other recessive trait, there is a 2/3 chance that the other recessive trait is present as well (in a heterozygous arrangement).

So, you can grow 8-12 plants, look for double recessives for either trait and collect all of the single double recessives with a very good chance that the second trait will show up phenotypically in 1/4 plants in the next generation.

This is a really helpful thing to keep in mind.

It works even better if you can save seeds from more than one F2 parent with one trait homozygous.