A 4-year-old boy presents with multiple fractures of the femur and humerus sustained from minor falls. Clinical examination reveals blue sclerae, translucent gray-brown teeth, and conductive hearing loss. Genetic testing of the parents' blood DNA is completely normal. The pedigree shows two unaffected parents with two affected children (both with identical COL1A1 mutations). The mechanism marked **A** in the pedigree diagram — germline (gonadal) mosaicism — explains this inheritance pattern. Which of the following statements best describes the genetic counseling implication for this family regarding future pregnancies?

Explanation

## Why "The recurrence risk is approximately 5–7%, as the mutation exists in a subset of parental germ cells despite absent somatic mutation" is right Germline (gonadal) mosaicism, marked **A** in the pedigree, is the presence of a mutation in a subset of a parent's germ cells (sperm or oocytes) WITHOUT presence in their somatic tissues (blood, skin). The parent is therefore phenotypically and genotypically normal on standard genetic testing of blood, yet can transmit the mutation to multiple offspring. This explains the "impossible" pedigree of unaffected parents with two affected children bearing the same dominant COL1A1 mutation. The empiric recurrence risk for germline mosaicism in osteogenesis imperfecta and other autosomal dominant conditions is 5–7%, not 0% (as a de novo mutation assumption would suggest), and prenatal testing should be offered for subsequent pregnancies (Nelson Pediatrics 21e Ch 743; Harrison 21e Ch 411). ## Why each distractor is wrong - **"The recurrence risk is 0%, as both parents have normal somatic DNA and neither carries the mutation in their blood cells"**: This ignores the critical concept of germline mosaicism. Although parental blood DNA is normal, the mutation persists in a fraction of germ cells, conferring a 5–7% empiric recurrence risk — not zero. This is the most dangerous misconception in genetic counseling. - **"The recurrence risk is 50%, as one parent must be a heterozygous carrier despite normal blood testing"**: A 50% recurrence risk applies to a parent who is a true heterozygous carrier (mutation present in ALL somatic and germ cells). Germline mosaicism involves mutation in only a SUBSET of germ cells, resulting in a much lower empiric risk of 5–7%, not 50%. - **"The recurrence risk is 25%, as both parents must be carriers of an autosomal recessive form of osteogenesis imperfecta"**: Osteogenesis imperfecta is autosomal dominant, not recessive. The 25% risk applies to autosomal recessive inheritance with two carrier parents. This distractor conflates inheritance patterns and misclassifies OI. **High-Yield:** Germline mosaicism = unaffected parents with multiple affected children (autosomal dominant disorder); normal parental blood DNA; 5–7% empiric recurrence risk; classic in OI, DMD, hemophilia, NF1, achondroplasia. [cite: Nelson Pediatrics 21e Ch 743; Harrison 21e Ch 411]