Help with some terminology

[MidwestMatthew]

I keep seeing the following terms showing up in rabbit forums and articles:

F1
F2
Terminal Cross

Google has been surprisingly unhelpful in finding a clear-cut explanation of what these terms mean. I haven't been able to find a definition at all for F1 and F2, and although I've read about "terminal crossbreeding," I have yet to see any explanation of what it actually is or why it's a good thing.

Would very much appreciate enough basic info on these terms to continue on my path of self-education!

 

[Bunnylady]

A terminal cross is one that isn't intended to produce breeding stock - all animals produced from a terminal cross are expected to be slaughtered. You cross this breed to that breed to get "hybrid vigor" and optimum production in the next generation, but that's as far as it goes.

When it was first developed, the Altex was used for terminal crosses. The developers only sold bucks, to be used over New Zealand or Californian does to produce fryers to be sold to meat processors. None of the offspring were saved for breeding.

If you breed two animals of different breeds (or possibly species) together, the offspring of that original cross are the F1 generation. An F1 hybrid could be sterile (a mule is the F1 hybrid of a donkey and a horse, for example), but if not, the product of breeding two F1 hybrids together is the F2 generation.

There are lots of F1 hybrids in chickens. Some are simply more productive than their parent breeds, and some (like the sex-linked crosses) produce male and female chicks that are distinctly different colors, making identification very simple from the point of hatching (useful if egg production is the end goal).

I know of at least one commercial chicken that is an F2 hybrid - two different breeds are crossed to produce the roosters, and a different cross is done to produce the hens. These two F1 crosses are bred together, and the chicks from that breeding are the end goal - they are hardier and out-produce all 4 of the parent breeds.

 

[Latestarter]

Hey Bunny... just curious and maybe I've got this all wrong, I am no geneticist by any stretch, but thought I had a handle on this...

I believe an F2 would be a cross between an F1 male and F1 female with 50/50 splits of the same 2 breed parents...

If you do breed 1 with breed 2 for F1a -and- breed 3 with breed 4 for F1b... then breed F1a with F1b wouldn't you get F1c not F2?

It would be after all the first generation of the combined 4 breeds, not the 2nd generation...

 

[Bunnylady]

I'm not sure; I may have pushed it a bit for completely different F1's but it doesn't have to be identical F1 hybrids.
This figure shows the breeding of the Altex. The F1 generation were all half Flemish Giant, but they weren't the identical cross - some were half Californian, and some half Champagne's. The result of this 3-way hybridization is labeled F2, not some variation of F1:
http://users.tamuk.edu/kfsdl00/altex-figures/figure1.jpg

(can't remember where I saw the description of the breeding of the chicken I referred to, but I'm pretty sure the article referred to them as F2 hybrids)

 

[Latestarter]

wow... maybe because there's a shared sire? Not even gonna try to figure it out...

 

[ragdollcatlady]

Not sure if this is any more helpful, but this is an example in goats, trying to breed mini nubians :

We start with a pure Nubian female x pure Nigerian male = F1 kid (lets call her Lolli)
Now we breed our F1 Lolli (50%/50%) x another F1 (50%/50%)= F2 kid (lets call her Pop)
Our F2 kid (Pop) is still 50% Nigerian and 50% nubian.
Now we breed our F2 Pop (50%/50%) x another F2 (50%/50%)= F3 kid (lets call her Sugar)
You keep going this way until you get to your registries definition of "purebred" or "american".

Example: if we were to breed an F1 x an F4, then the kids would be F2.
Your lowest numbered "F" parent is the limiting factor.

Different example: if we breed an F3 to a pure Nubian, then our kids will be F1 but the percentages will be different, we will have 50% nubian from our pure parent and (if our F3 was a 50%/50%) add 25% nubian and 25% nigerian from our F3 parent. So this new kid is actually 75% nubian and 25% nigerian.

Was this helpful?

 

[Latestarter]

That's the thing... I understand that example completely because we're working with only 2 breeds... all the way down the progression. The issue was when you start adding in a 3rd and 4th breed...

So if you breed an F2 that's 50% Lamancha and 50% Nigie with an F2 that's 50% Boar and 50% Nubian, what would you have?

It would be the first generation combining all 4 breeds, so would it drop back to an F1, or would it be an F3 because both were F2s?

In order to progress down/up the F scale, both parents need to consist of the same breeds as the initial breeding or some percentage of them.

 

[Bunnylady]

Here's a (tiny) fragment of one of the articles that I have dug up regarding broiler chickens:

# Parent stock which are achieved by crossing the two male lines to achieve the parent male and by crossing the female lines to achieve the female. This then becomes two distinct F1 hybrids with no inbreeding from opposing strains.

# Specific broiler chicken produced by crossing the parent male and female which is an F2 hybrid.

The article is very long and rambling, but the author is clearly describing 4 different strains (breeds), 2 of which are crossed to produce the male parent, and 2 others which produce the female parent, with a 4-strain cross broiler being the end goal, which is termed the F2 generation. In this example, the two F1 parents are in no way related, and the offspring are considered F2 hybrids.

And y'know, while poking around looking for something I could post regarding the F2's, I came across something talking about this regarding dog breeding, specifically Labradoodles. It got me to wondering - if you crossed a Labradoodle with a Cockapoo, would the results be a Cocklapoodledoo?

 

[Latestarter]

 

[MidwestMatthew]

So it sounds like the F levels could be described as "levels away from purebred." Is that accurate?

And "terminal cross" appears to simply mean "I do not intend to breed this animal." Is that correct?

In that case, what would make a terminal cross a good thing? Not that I'm arguing it's bad, but why does it seem to be a "best practices" thing?