[oldcow]

I had read several times on this forum that if two varieties of a plant cross pollinate (say two types of peppers or of tomatoes) the fruit will be true to the plant it grows on, but if you were to use the seeds from that fruit you'll get the effect of the cross pollination on the new plant.
Now, I have two types of tomatoes growing one next to the other. One is the Costoluto Fiorentino, a red, ribbed, large beefsteak tomato. The other one is a Pink Dr Carolyn, which is medium sized, round and smooth. Basically, two very different looking tomatoes in every single aspect.
This morning I have picked up from the Dr Carolyn plant a very large ribbed beefsteak tomato, looking just like the Costoluto, but with the exact pink colour of the Dr Carolyn tomatoes. What gives? Did anybody else have the same thing happen to them?

[Swing Swang]

Some possible explainations:

Contaminated seed - I once had a Bolle de Fuocuo (can't remember spelling) in a batch of Costoluto Fiorentino from Real Seeds (they were very apologetic and offered a refund, but I refused, saved the seed, and that's all I've grown this year, very tasty and very prolific bush toms that cook really well).

Cross pollination of the parent plant (in other words the seed wasn't true)

Seed from an 'undeveloped strain' - seed is selected for many generations of 'parent' plants before it is released to the market, but there may still be some genetic variation (particularly in new, or less rigourously screened varieties) and 'throwbacks' are possible. Edit - this applies to open pollinated varieties, not F1s

Genetic mutation

[mumofstig]

and yes you do occasionally get a weird tomato on an ordinary plant..........usually the result of a weird or extra-large flower. The resulting fruit is often catfaced.... it looks as if it has a weird twisted 'belly button' on the bottom, for want of a better description

[Swing Swang]

@oldcow

Out of interest are all the fruits on the plant similar, or is it a single 'rogue fruit'? - Didn't think this one through when I replied whereas mumofstig did.

SS

[oldcow]

It's one truss that has the one big unusual looking tomato, and the other tomatoes on the same truss have the Dr Carolyn colour and size but are slightly ribbed instead of being smooth as all the other ones on the plant. So it looks like I have a "mutant truss"...
The variety is open pollinated, but I thought that if the seeds weren't true all the tomatoes on the plant would look similar.
It's not a "conjoined twin tomato", I have had a couple on other plants and I know what you mean, but this looks just like a normal, big beefsteak and on the bottom it has just a long straight line.

[Swing Swang]

If you like them then keep some seed and see what happens next year...