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Achondroplasia is a form of dwarfism that is characterized by short limbs, normal-sized abdomen, torso, and neck, and macrocephaly.

Could there be a form of gigantism that is characterized by long limbs, normal-sized abdomen, torso, and neck, and microcephaly?

I ask because I am writing a story with a character having "hyperchondroplasia".

If this genetic disease exists, what is its mode of inheritance? Autosomal dominant (like type 1 von Willebrand disease), autosomal recessive (like cystic fibrosis), X-linked dominant (like X-linked pituitary gigantism), X-linked recessive (like Hunter syndrome), incomplete dominance (like myostatin-associated muscular hypertrophy), or mitochondrial (like Leigh syndrome)?

If this is autosomal, what chromosome is linked? Chromosome 1 (like Hutchinson-Guilford progeria), chromosome 2 (like Ehlers-Danlos syndrome), chromosome 3 (like retinitis pigmentosa), chromosome 4 (like Huntington disease), chromosome 5 (like Sotos syndrome), chromosome 6 (like hemochromatosis), chromosome 7 (like tritanopia), chromosome 8 (like Werner syndrome), chromosome 9 (like cartilage-hair hypoplasia), chromosome 10 (like type 2 multiple endocrine hyperplasia), chromosome 11 (like sickle-cell anaemia), chromosome 12 (like phenylketonuria), chromosome 13 (like Wilson disease), chromosome 14 (like Krabbe disease), chromosome 15 (like Tay-Sachs disease), chromosome 16 (like Morquio syndrome), chromosome 17 (like type 1 neurofibromatosis), chromosome 18 (like type-C Niemann-Pick disease), chromosome 19 (like Donohue syndrome), chromosome 20 (like adenosine deaminase deficiency), chromosome 21 (like autoimmune polyendrocrinopathy-candidiasis-ectodermal-dystrophy), or chromosome 22 (like type 2 neurofibromatosis)?

I know I made an extremely long enumeration because the vast majority of the human genome is nuclear and autosomal, sorry.

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    Shouldn't you see if the word exists before you commit to using it in a book? Dr. Google doesn't seem to know that word other than a few misspellings of hypOchondroplasia. If the condition existed, I think I'd find more than that.
    – Carey Gregory
    Nov 30, 2021 at 17:43
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    @CareyGregory I see the term formerly existing. There's even a (former) medical condition "hyperchondroplasia auris traumatica", AKA "cauliflower ear", it also used to be used for Marfan syndrome as Marfan's, and I see it as recently as 1999 for that.
    – bob1
    Dec 3, 2021 at 3:04
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    @bob1 Yeah, I saw the same references, but the word doesn't seem to be used currently the way OP wants to use it, and in the past it didn't mean what OP intends. In any case, I don't think the condition OP is imagining exists no matter what you call it.
    – Carey Gregory
    Dec 3, 2021 at 4:56

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Achondroplasia is caused by very specific genetic changes of the FGFR3 gene encoded on chromosome 4.

According to the GeneReviews on Achondroplasia:

The p.Gly380Arg pathogenic variant resulting in achondroplasia causes constitutive activation of FGFR-3

The section on other disorders caused by the same gene lists:

CATSHL syndrome (camptodactyly, tall stature, hearing loss) (OMIM 610474), an overgrowth disorder caused by pathogenic loss-of-function variants in FGFR3

So in fact, there is a disorder which is caused by the same gene, but inactivation rather than activation. However, it is called CATSHL, and is not exactly as you have described. It is inherited in an autosomal dominant pattern.

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