Final answer:
Sally's brown hair, determined by her Bb genotype, represents a dominant trait because the presence of the dominant allele (B) is sufficient to express the brown hair phenotype.
Step-by-step explanation:
If Sally has brown hair and her genotype is Bb, she exhibits a dominant trait. This is because the presence of a dominant allele (B) is enough to show the corresponding phenotype, which in this case is brown hair. The recessive trait would only be exhibited if she had two recessive alleles (bb).
In genetics, a dominant trait is one that is expressed in the phenotype of an individual carrying at least one dominant allele. For example, the Punnett square shows that crossing two heterozygotes (Bb) will result in offspring that express the dominant phenotype in a 3:1 ratio compared to the recessive phenotype. This indicates that in presence of a dominant allele, like B for brown hair, an individual will exhibit that trait over the recessive trait (white hair). Hence, the correct answer is A) Dominant.