Question about grex names.
 

I am slowly learning more about what a grex means. Initially, I thought it was a shorthand for the entire pedigree of an orchid, but found I was somewhat mistaken.

What if we do crosses, (AxB)x (CxD) and also (AxC)x (BxD)? The progeny of both crosses will have 25% parentage from each of parents A,B,C, and D. Would these be considered the same grex?

And if they are not, is there a name for this relationship?

Also, would we expect or not expect the progeny to be just as similar as siblings?

No they would not be the same grex.

When listing a cross as (AxB) then A is the plant that holds the pod and B is the plant that supplies the pollen. Once a cross has been registered as (AxB) then any future crosses of either AxB or BxA would have the same grex. These crosses can and do vary wildly which is why it is difficult to identify flowers once a label is lost.

Existence of a grex name depends on the identity of the two immediate parents, not the ancestry farther back. The person flowering the first plant from a grex, the first time, is usually the one to name it. All further plants produced from crossing those two parents, in either direction, bear the same grex name.

Reciprocal crosses are interesting because certain characteristics are passed along more strongly by pod or pollen parents. In some cases A x B and B x A yield two very different groups of offspring, all with the same grex name.

For example, people with limited growing space often search for grexes with the smaller parent as the pod parent.

The others covered the basic question. The situation you described would not be the same grex.

There is no special name for this relationship, either in orchids specifically or in plant breeding in general, as far as I'm aware.

If you used the exact same plants of each species, and particularly as you specified the cross with the same plant A as the mother line (the pod parent of the pod parent) in each case, the 2 populations of seedlings could be very difficult to distinguish, showing much the same range of variation, though there would be some exceptions. In the first case you could get seedlings that look a lot like AxC for instance, but the second case you would not. And within each group of seedlings (each set of true siblings) there could be seedlings that hardly look alike at all. It would also depend on how similar the 4 species are to begin with.

But as each species (A, B, C and D) could be quite variable on its own, if different plants of each species were chosen in each case, the 2 sets of seedlings could be quite different.

First of all, there is a big difference in dominance, depending upon whether we are talking about primary hybrids or complex hybrids.

Secondly, when it is a primary hybrid, there can be very clear differences between AxB and BxA. Various traits dominate differently; for example:

Paph insigne x Paph villosum = Paph Nitens

Paph insigne dominates for color, in both cases.
Pod parent dominates for shape, in both cases.

I have bloomed Renanetia Sunrise (Renanthera imschootiana x Neofinetia falcata) bred both ways. The pod parent totally dominates both the shape of the flower and the plant structure.

When it comes to breeding hybrid x hybrid, there is a broad spectrum of results, which makes it much harder to predict outcomes.

Just to mention...

a primary cross is a cross between two species, and a complex cross is a cross between hybrids that have various species in their background.

So obviously, there would be more variation in a cross involving parents with a complex background, so progeny can vary wildly (dependent on stable dominant traits of course).