Small necro of my own thread.
As I've continued poking around the boards and the interwebz, I've probably confused myself more than I already was.
So, let's go with a more specific hypothetical. Clan Snow Raven, with a warrior that is eligible for the McKenna General Bloodname, and is a Aerospace Pilot.
Using this an example since I read a bit of ER 3145 and Sterling McKenna's entry - she appears to have the eyes of an aerospace pilot and was a notable pilot in practice - but, the entry makes it seems like she became an aerospace pilot because she tested well and showed an aptitude for it, as opposed to being "bred" specifically for it.
For years I was under the impression there was a certain amount of genetic engineering vs. targeted breeding, but the more I'm reading the more I think I was mistaken.
So conceivably, in a xyz-McKenna sibko, there could be a hundred kids, and they potentially have no idea if they'll end up as pilots, mechwarriors, elementals, etc until they grow a bit, start to develop those traits/markers, and test for certain fields?
I mean I'm sure some will have an idea early on, like Elementals, but couldn't this also mean that you could potentially end up with Mechwarriors that have some "defining" aerospace pilots traits like the larger eyes, or larger-than-average Aerospace pilots because an individual tested exceptionally well in one field or another?
Generally speaking, what I've always seen in every book/source I've ever read and I've studied the Clans quite a bit. Individual trueborn Cadets are brought with a specific phenotype and thus role (i.e. mechwarrior, ASF pilot etc) in mind.
That's the norm.
Then we see in some Clans, like the Horses and Bears, if they fail their first ToP they may be routed to a different warrior subcaste to see if they do better there.
But we do see odd little exceptions and situational exceptions. It's important to think of these things as general guidelines and not absolutes.
To use a Bears example, in FM: Warden Clans on page 90 the commander of the 140th Strike Cluster, Boris Jorgennson. A sibmate of Khan Bjorn. After watching his sibmate's ToP in ASFs (he failed his first as a mechwarrior), Boris voluntarily resigned his warrior status (meaning mechwarrior status) and retested as a pilot as well.
Here's another one. Among Clan Wolf in Exile, in FM: Warden Clans page 158 we get Galaxy Commander Craig Fetladral. He is an Elemental turned mechwarrior. He sustained injuries that made it impossible to wear the armor. So he became a mechwarrior. He might have just been granted permission to do that. But I could easily imagine him launching a Trial of Possession for the right to train as a mechwarrior.
Regarding mixed phenotypes, we've seen the Horses experiment with tankwarriors. We've seen the Cloud Cobras experiment with combining aspects of Elemental and ASF phenotypes to create Elementals suited for flying Elemental suits like the Sylph.
So here and there we get these little examples that tell us that for the most part, Trueborn warriors are bred with a specific phenotype in mind. But there are situations where they diverge from that. They are notable enough that they tend to get special mention, thus likely rare.
Now your particular example about Sterling McKenna comes with a lot of question marks. In the new era, much has changed. I'm not sure which Clan practices may have changed (or at least been experimented with).
But there's a specific issue with Sterling McKenna. The Snow Ravens have the naval bloodline, and train naval warriors straight from the sibko.
It's been said, thinking of FM: 3145 in particular, that in this new era, the Snow Ravens are reactivating their fleet (warships, jumpships, dropships).
That means they suddenly need a lot of ship crews, including warrior officers. And it takes time to just breed new naval sibkos from scratch, about 20 years, too much time.
The Snow Ravens naval bloodline/phenotype is considered a sub-category of aerospace phenotype.
** Warning, from here, this is really just my speculation****
So its very possible that Sterling McKenna got caught up in a canvassing process of the warriors and/or sibkos to look for suitable personnel to shift to the naval branch. But due to those attributes mentioned, she didn't get routed to a naval track, she stayed on an aerospace pilot track.
It's also possible that the aerospace and naval bloodlines (and phenotypes) are so similar, that in this case the cadets are considered to be possibly suitable for either role, and are evaluated and then routed accordingly based on early training and aptitude results. That might be standard procedure in this new era, or a situation specific to the 3145 era and this Clan's naval reactivation.