The tomatoes above are F2 Galahad tomatoes. They have long brown scars. What causes these scars?
Most F2 plants show no scars.

Cracking in tomato skins happens in adverse weather conditions, but there is a genetic component, as there is in splitting.
It seems to me that the growth rate of the skin cannot keep pace with the expansion of the fruit in certain conditions.
In some cases shape genetics seem to play a role as well as the thickness or pliability of the skin.
Therefore you get cracking or splitting. The skin is lacking elasticity or is too thin. Or it is hardened by weather conditions and when the fruit expands, it cracks.
If cracking or splitting is a problem, it is best to select away from it in your F2, if possible.
Then again, if the weather was very stressful and the only good tasting selections have cracks, you might keep them and select away from cracking in a future generation.

In papers I read the most scar resistant genes are recessive. And scar resistance is modified by 2 or 3 main genes.

If the F1 do not have scars, most of the F2 does not have scars and i take F3 seeds from a plant with scars. Can I still get F3 tomatoes resistant to scars?

If the genes involved are recessive then your plant with scars will have future generations with the same genotype and phenotype.
I would not forget that environmental conditions play a role, and perhaps in other conditions you wouldn't get the scars. But for safety sake you would be better to select a different plant, if indeed this pheno indicates double recessive genetics.
I wasn't aware of these genes being recessive, and not sure whether "scars" really covers the cracking issue which IMHO is complex and involves variables including skin thickness and elasticity, and minute details of shape - which again can be complex and involving multiple genes as well as interactions with environment.
I selected an F5 for seed this year based on taste, but a couple of the fruits did have some cracking, which is unusual in that line. Will be growing ithe F6 next year for sure, so I'll be watching to see if cracking turns up again, and whether it affects all plants in the next generation.

I found this full text article with some interesting references on cracking genetics:
https://scialert.net/fulltext/?doi=ajar.2017.10.17
The literature seems to be split as to the indication of recessive, partly dominant, or completely dominant genetics, with effects that are additive and a high probability that some of these traits are QTL's (pericarp thickness for example, very likely QTL's involved.)
With that in mind, the possibility of crack resistant fruit in your F3 is certainly reasonable.

I am seeing some concentric scars and smaller fruits on my Big Beef now that it is the end of the season and we have also had lots of rain.

There may be a genetic component but I think that weather conditions have more to do with it.

Big Beef is an F1 hybrid and I grow it every year from fresh seed. I do not save seeds.

I've always wanted to ask about this. I get scarring only in the fall. Sometimes no matter how careful I am with watering consistency.

From what I've read, it does seem to show up more in the fall. I wonder if it's because the root system is at it's largest size, and more water can be absorbed than earlier in the year.

I'll take concentric cracking over radial cracking. Concentric cracking is usually around the stem area, not the most luscious part of many tomatoes.

And, if you really want to go for what might be maximum cracking effect, try Soldacki. In my garden. it consistently gets both radial and concentric cracking at the same time. The cracks intersect, creating a grid where the tomato looks like a pink mini disco ball. The flavor is so good, I put up with the possible, but not always there, cracking

Other F2 plants do not show the scars. but the taste of the one with the scar is much better. Only i do not want a tomato for outside that is not crack resistant.

Well in that case, I would save seed from the best tasting one with the cracks, and save seed from the second best tasting one without cracks. Grow the first one and maybe you will find crack resistance. Grow the second one and maybe you will find the preferred taste.

I wonder if something that makes the fruit crack resistant, also changes the taste to something less appetizing.

@Rockoe10 that would be a genetic nightmare.
If the cracking fruit is so different tasting from the others, and continues to excel for taste in the next generation with cracks present, @Doffer could backcross it to a non-cracking sibling, and then select for a non cracking descendant with the better taste.
Assuming that the crack genetics are recessive and homozygous, the backcross would create a path to select it out, without too much alteration of the desired genetics in the line.
The real problem with taste is that it is such a complex trait.
Some lines seem to have very robust taste genetics, so that all or most of the plants have great tasting fruit through the generations.
When crosses have really divergent taste profiles in the parents, it's another story. Results can be all over the place. This is where you need to grow lots of plants every generation to find and select the taste you wanted.

I think next year I will sow from the 3 best plants with different characteristics.
All have a high brix and taste good.

It will then be about 100 F3 tomato plants for 2022.

The first plant (F2 plant 31) advantages:
is the tastiest (brix 6.2), pink, good vigor, good size fruit.
The first plant (F2 plant 31) cons:
seems prone to cracking, green shoulders.
The second plant (F2 plant 21) advantages:
is also tasty (brix 6.0), red.
The second plant (F2 plant 21) cons:
medium vigor, smaller fruits and green shoulders.
The third plant (F2 plant 42) advantages:
has a brix of 7.0, red, uniform green.
The third plant (F2 plant 42) cons:
Very weak vigor, smaller fruits, late production

O and dont forget, all Galahad tomatoes are blight resistant with both ph2 and ph3 genes homozygous

Rockoe10 wrote: ↑Wed Sep 29, 2021 12:24 pm I wonder if something that makes the fruit crack resistant, also changes the taste to something less appetizing.

Actually, cracks in other fruits are known to make them tastier: e.g. netting on cantaloupes, corking on peppers, etc. Dried up cracks might do the same with tomatoes.

Shule wrote: ↑Sat Oct 02, 2021 7:39 am

Rockoe10 wrote: ↑Wed Sep 29, 2021 12:24 pm I wonder if something that makes the fruit crack resistant, also changes the taste to something less appetizing.

Actually, cracks in other fruits are known to make them tastier: e.g. netting on cantaloupes, corking on peppers, etc. Dried up cracks might do the same with tomatoes.

Interesting! If this is true, then I wonder what the effects would be of heavy watering early on, which would cause scarring, and drought later on, which would increase sugar ratio.