Okay, coming up for air (though may get pulled back into the depths at a moments notice…)
So… I apologize in advance but this is probably going to be a bit heavy on the jargon.
Without an actual gene-level interrogation, we do not know the identity of the gene at play in this complex but the consensus among those of us in the field is that it is most likely something to do with melanocyte migration and neural crest development. Background relevance here… Melanocytes, in addition to their well known, more normal role in being the cell type that holds melanin, also sometimes act as sort of “guides” for other cells to follow from point A to point B. When it comes to the neural crest, the proper distribution and integration of subsets of nerves is dependent on the proper migration of the melanocytes.
To try and make this an analogy - Imagine you have to run new fibre-optic cable under a street to wire up some houses. In a normal (WT/WT) case, you have a robotic drill unit that has a pair of drill heads that work together to bore a nice clean round hole under the street and the branch points to each of the houses and you can easily feed the fibre-optic cable through it.
With the majority of the mutants in this complex (Spider, Champ, Woma, HGW), one of the two drill heads in the pair is smaller/missing. So instead of a nice clean round hole you end up with a smalled, ragged, rough, spiraling hole. This makes it much harder to feed the fibre-optic cable through - the normal amount of cable gets hung up or kinked or forms clumps so you have to halve the amount of fibre-optic cable you put through so the performance in the houses is much reduced (manifest as wobble/neuro/twitch)
With the Blackhead mutation, we see something different. In this case, one of the drill heads is now larger than the other. The hole you end up with may be slightly rougher than normal but it is significantly larger and because of that you can actually feed extra fibre-optic cable through the hole. In this case, the performance in the houses is increased.
Now, when you pair Blackhead with one of the other mutants, you essentially end up with a hole that may not be perfectly circular but is still the proper sixe and because of that you can actually the normal amount of fibre-optic cable through the hole and end up with normal performance in the houses.
And, by extension, if you have a superform of the bad mutations, both of the drill bits being under-sized/missing means that you cannot feed sufficient fibre-optic cable through the hole for the houses to function at all
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I hope that makes sense