If both parents are affected and have an unaffected child, the trait is
If trait were recessive, two affected parents homozygous recessive aa would transmit only recessive a allele to every gamete, making all children homozygous recessive affected without exception. Observation of phenotypically unaffected child from two affected parents directly contradicts recessive model expectation. For dominant inheritance, affected individuals may be heterozygous Aa, carrying one normal allele that can be passed to child yielding normal aa phenotype. Thus presence of unaffected offspring in cross of two affected parents indicates dominant allele where parents are heterozygou
Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 2: Distinguishing Dominant vs Recessive
