Final answer:
If two unaffected parents have an affected child, it implies both parents are carriers of a recessive trait. This pattern of inheritance is typical of autosomal recessive disorders, where an affected individual must inherit the recessive gene from both carrier parents.
Step-by-step explanation:
In pedigree analysis, if two unaffected parents have an affected child, this indicates that both parents are carriers for the trait, meaning the trait is recessive. Carriers do not show symptoms of an autosomal recessive disorder because they have only one copy of the recessive gene, and the normal allele compensates. However, when two carriers have a child, there is a 25% chance the child will inherit the recessive gene from both parents and express the disorder. This situation does not occur in autosomal dominant inheritance patterns, as only one copy of the dominant allele is required to express the trait.
Carrier parents have a genotype of Aa, where 'A' is the normal allele and 'a' is the disease allele. The possibilities for their offspring's genotypes follow Mendelian inheritance principles, as seen in the Punnett square: there's a 25% chance of having a child with aa (affected), 50% chance of Aa (carrier, unaffected), and 25% chance of AA (non-carrier, unaffected).