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Old 05-24-2015
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Default NBME 15 question about osteogenesis imperfecta

A couple has two children with osteogenesis imperfecta type II. (OI). Genetic analysis of the 42-year-old father shows that 1 in 10 of his sperm carry mutations of the type I procollagen genes associated with OI. Which of the following is the best explanation for this finding?

A) Advanced paternal age
B) Consanguinity
C) Do novo mutation
D) Genomic imprinting
E) Germline mosaicism

Can someone please explain why it is E? I was going to pick E at first but since it said 1 out of 10 sperm I thought it was wrong....I thought if there was a germline mutation that a larger fraction should have this defect so I choose A (thinking that achondroplasia has a relationship with paternal age and gene mutation so...) Obviously I am not understanding germline mosaicism....I thought that half or at least more then 1 out of 10 should have the mutation....Can anyone clarify this subject for me? Thanks in advance! Good luck to all!
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