Two siblings were diagnosed with Osteogenesis imperfecta. But their parents are normal. Mechanism of inheritance is:
**Core Concept**
Osteogenesis imperfecta (OI) is a genetic disorder characterized by brittle bones, blue sclerae, and other skeletal abnormalities. It is caused by mutations in the COL1A1 or COL1A2 genes, which encode for type I collagen. This condition is inherited in an autosomal dominant pattern, meaning a single copy of the mutated gene is enough to cause the condition.
**Why the Correct Answer is Right**
The correct answer is because OI is caused by mutations in the COL1A1 or COL1A2 genes, which are located on autosomal chromosomes (chromosomes 17 and 7, respectively). In an autosomal dominant pattern, a single copy of the mutated gene is sufficient to cause the condition, and the mutated gene will be expressed in the offspring. This means that even if the parents appear normal, they can still be carriers of the mutated gene and pass it on to their offspring.
**Why Each Wrong Option is Incorrect**
**Option A:** Not applicable, as this option is missing.
**Option B:** This option is incorrect because OI is not inherited in an autosomal recessive pattern. In autosomal recessive inheritance, an individual must inherit two copies of the mutated gene (one from each parent) to express the condition.
**Option C:** This option is incorrect because OI is not inherited in an X-linked dominant pattern. In X-linked dominant inheritance, the mutated gene is located on the X chromosome and only affects females, who have two X chromosomes.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that autosomal dominant conditions can be inherited from an affected parent or can occur de novo (without a family history). This is a common exam trap, and knowing the difference between autosomal dominant and autosomal recessive inheritance is crucial for clinical diagnosis.
**Correct Answer: D. Autosomal dominant inheritance.**