Perhaps I should give some background on Osteogenesis imperfecta (OI). It is caused (usually) by a deficiency in type-I collagen (C1). While it is often called “autosomal dominant inhereted,” this is misleading as de novo mutations account for a large quantity of the cases. So now let’s imagine we’re talking about Jell-O again. I’ll explain what these mutations would do to Jell-O.
Type I OI results from a mutation in which only half the collagen a cell would typically make is produced. Think Jell-O with half a the ammount of powder…
Type II OI results from a mutation in which one (or more) of the nonpolar amino acids (glycine) is replaced by another (this time mildly polar such a cysteine) amino acid or is completely missing. This results in bonding between chains being weaker. Think of when you’re making Jell-O shots how you have to use more Jell-O and less liquid, or alternatively cool them much more than with water. This is because the EtOH (ethanol) in that vodka interferes with these same interactions. The worst of type II results from a mutated C-terminus (the end of the protein chain) which results in the chains not being able to associate end-to-end.
Type III OI results from completely missing segments of the pro-C1α1 (or absence of C1α2) or one (or more) glycine being replaced by a polar amino acid (such as glycine). This results in the triple-stranded mature collagen being unable to form. Your Jell-O has been completely dissolved in hexane, you just have collagen blobs floating around. It’s essentially a non-newtonian fluid where, in some conditions (high polarity solutions such as within bones) you can have weak interactions of collagen.
Type IV OI results from a completely screwed up post-translational modification of the propeptide for C1α2 (~10 amino acids are knocked out from the helical domain) which results in malformed collagen. The Jell-O model fails this one, the interactions aren’t going wrong, the molecule is misshapen.