All are seen in osteogenesis imperfecta except-

A molecular defect in type I procollagen can be detected in 2/3 of patients by incubating skin fibroblasts with radioactive amino acids & then analyzing the pro a chains by polyacrylamide gel electrophoresis. It shows : Decreased rate of synthesis of pro α1 (1) chains relative to pro α2 (I) chains Abnormally long / short pro a chains Pro a chain with extensive post-translational modifications as amino acid substitution impairs folding of the triple helix. Mutations are defined by sequencing of genomic DNA. Exact mutation is identified by using 100 polymerase chains reactions (PCR) to analyse 10,000 bases in each of two collagen genes. After a mutation in type I procollagen gene is identified, a simple PCR test can be used to screen family members at risk or for prenatal diagnosis. Chorionic villi biopsy at 8-12 weeks demonstrates synthesis of abnormal pro a chains.