Achondroplasia is a form of dwarfism that is characterized by short limbs, macrocephaly, and normal-sized abdomen, torso, and neck.
Could there be a form of gigantism that is characterized by long limbs, microcephaly, and normal-sized abdomen, torso, and neck?
I ask because I am writing a story with a character having "inverted achondroplasia". If this hypothetical syndrome exists, is it dominant, recessive, autosomal, x-linked, y-linked, or mitochondrial?
While there are existing types of gigantism that only impact parts of the body, such as acromegaly or the aptly-named local gigantism, none of them create the physiology specified - long limbs with otherwise normal proportions. Klippel–Trénaunay syndrome can cause hypertrophy of the limbs, but that only increases volume, not length.
Since (based on quick research) there is no existing form of gigantism that causes this, we can look at other genetic conditions that cause elongated limbs. Marfan syndrome (https://en.wikipedia.org/wiki/Marfan_syndrome, a surprisingly good overview) seems like the closest fit. While individual cases vary, some with the condition can grow taller than average with longer limbs.
So, it is conceivable that either Marfan syndrome or something similar to it would create the effect you are looking for. Marfan itself is an autosomal, dominant mutation in the FBN1 gene (https://omim.org/entry/134797). Not strictly speaking gigantism, but it is the closest real-world condition I am aware of.
