A 3 year old girl is found to have recurrent fractures after tril trauma. On taking a detailed history, several other family members were found to be affected by similar complaints. So, a pedigree cha was drawn, which is shown below. Which of the following is the mode of inheritance of this disorder?
First, I need to think about genetic disorders that cause bone fragility. Osteogenesis imperfecta (OI) comes to mind. OI is known for causing brittle bones and is often inherited. The question mentions a pedigree, so the pattern of inheritance is crucial.
The options aren't listed, but common inheritance patterns include autosomal dominant, autosomal recessive, X-linked dominant, and X-linked recessive. Let's consider OI's inheritance. The most common type, Type I, is autosomal dominant. This is because it's caused by mutations in the COL1A1 or COL1A2 genes, which are located on autosomes. Since the question states that multiple family members are affected, an autosomal dominant pattern would explain this. In autosomal dominant disorders, only one copy of the mutated gene is needed to cause the condition, so each child of an affected parent has a 50% chance of inheriting the mutation.
If the inheritance were autosomal recessive, both parents would need to be carriers, and the pedigree would show affected individuals with unaffected parents and siblings. X-linked recessive would affect more males, and X-linked dominant would affect both genders but with different expressions. The question mentions a female patient, which might make X-linked recessive less likely since males are more severely affected in those cases.
So, the correct answer is likely autosomal dominant. The pedigree would show affected individuals in each generation, with both males and females affected, and no skipping of generations. The key here is the presence of multiple affected family members across generations, which aligns with dominant inheritance. Also, OI Type I is a classic example of an autosomal dominant disorder presenting with these symptoms.
Clinical pearl: Remember that OI is a classic autosomal dominant disorder with bone fragility. When you see multiple family members with similar issues, especially in a non-X-linked pattern, consider autosomal dominant inheritance. The presence of affected males and females in every generation is a strong indicator.
**Core Concept**
This question tests understanding of genetic inheritance patterns in disorders like osteogenesis imperfecta (OI). OI, characterized by brittle bones and fractures, is typically caused by mutations in collagen genes (COL1A1/COL1A2) and follows an autosomal dominant pattern.
**Why the Correct Answer is Right**
Autosomal dominant inheritance requires only one mutated allele to manifest the disorder. In OI Type I, affected individuals inherit one defective collagen gene from an affected parent. Collagen type I deficiency weakens bone matrix, leading to fragility. The pedigree would show vertical transmission (affected in every generation), with both sexes equally likely to inherit the mutation.
**Why Each Wrong Option is Incorrect**
**Option A:** *Autosomal recessive* is incorrect because it requires two mutated alleles. Pedigrees would show unaffected parents and sporadic affected individuals, not multiple generations.
**Option B:** *X-linked recessive* is unlikely because females (like the patient) would need two mutated X alleles to show symptoms, which is rare unless the father is affected and the mother is a carrier.
**Option D:** *X-linked dominant* is inconsistent with the described family history; it