Sarah can't curl her tongue - which is a recessive trait. both of her parents can curl their tongues - which is dominant. give the genotypes of all persons involved (use t and/or t in the genotypes).

Let T be the dominant allele, allowing the person to curl their tongue and t be the recessive allele preventing the person from curling their tongue.
In order for Sarah to inherit the recessive phenotype, despite her parents both having the dominant phenotype, her parents must be carriers for the recessive allele. Therefore, the genotypes are as follows:

Father: Tt
Mother: Tt
Sarah: tt