Severe forms of osteogenesis imperfecta (types II and III) are inherited in which pattern?

• A. Autosomal dominant
• B. Autosomal recessive
• C. X-linked recessive
• D. Mitochondrial

Answer: (B)

Severe forms of osteogenesis imperfecta, such as those described as type II and III, arise when the problem isn’t a mutation in the collagen chains themselves but in genes that help make or process collagen correctly. These genes, like CRTAP, LEPRE1, and PPIB, when mutated, disrupt collagen folding and maturation in a way that produces a very brittle skeleton. Because the defective function comes from both copies of such a gene (one from each parent), the pattern is autosomal recessive. Carriers usually have no symptoms, while an affected child inherits two mutated copies and has the severe phenotype. In contrast, many milder forms come from autosomal dominant mutations in COL1A1 or COL1A2, where a single mutated allele can cause disease.