Final answer:
Tyrion Lannister has achondroplasia, which causes his short stature, short limbs, and average-sized head.
Step-by-step explanation:
Tyrion Lannister displays the characteristics of short stature, short limbs, and an average-sized head, which is the result of a genetic condition called achondroplasia. Achondroplasia is a form of dwarfism that affects the growth of the long bones, such as those in the arms and legs. It is caused by a mutation in the FGFR3 gene, which regulates bone growth.