The mutation that causes achondroplasia is in the fourth chromosome. It is called the fibroblast growth factor receptor-3 gene (FGFR3). Two mutations in the FGFR3 gene is what causes 99% of achondroplasia. The FGFR3 genes job is to create the fibroblast growth factor receptor 3 protein. This protein is part of a group of proteins that help important cellular processes, such as the regulation of cell growth and cell division,determination of cell type, formation of blood vessels, wound healing, and embryo development. What causes the skeleton to be weak is that when an infant, the cartilage should convert to bone. That is the problem with people with the disease.
The yellow arrow is pointing to the area where the FGFR3 gene is on chromosome 4