How can a mutation in a tumor-suppressor gene behave as a recessive allele at the cellular level, but appear as a dominant allele in pedigree analysis?

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Tumor-suppressor genes typically require both alleles to be mutated or inactivated for a tumor to develop (a situation described as loss of heterozygosity). In a heterozygous state, one functional allele can often prevent tumor formation, acting recessively at the cellular level. However, in familial cancer syndromes, inheriting one mutant allele predisposes individuals to cancer, making it appear as if the mutation behaves dominantly in pedigree analyses. This is because a second, somatic mutation in the remaining functional allele can easily occur in any cell, leading to cancer.

How Can a Mutation in a Tumor-Suppressor Gene Behave as a Recessive