This question has been bothering me for quite some time. I want to know what the voices not in my head think of my question. An Andorian/Vulcan child? What do you think it would look like? Would it have antennae? Would its blood be blue or green? Blue or greenish skin? What species would have the dominant genes? What do you think?
As for the point about interspecies breeding being absurd, I do agree that it is a little far-fetched that humans and species just happen to share enough genetic similarity to produce healthy children. This could go back to the fact that all humanoids came from a single species (a TNG episode whose name I can't remember either).
Yeah, a single prokaryote. "The Chase," while well-intentioned, purported to explain humanoids, but all it did was explain the universal use of DNA as a genetic coding language, which is impressive and would have been cool had they known what they were doing.
I don't think the writers actually knew how eukaryotes developed. Hint: DNA played a subsidiary role.
We know all humanoids (well, most) in Trek came from the ancient humanoids, which is why they can interbreed. The same genetic engineering that made them walk upright, have opposable thumbs, two eyes, etc., could also have given the DNA 'instructions' as to how to create life with a different set of DNA, telling it what traits would exist, which would be dominant, whose blood type would flow in the child, what the skin color would be...just like two humans creating a child.
Except bacteria don't have opposable thumbs. Or mitochondria or cholorplasts. Or nuclei. Strangely, life forms that haven't changed as much in the intervening four billion years ago aren't as complicated. I wonder where that complexity came from, because it didn't come from the Salome Jensians.
Also, I suspect the immune system would have something to say about alien cum.