Recessive traits are characteristics that are not expressed in an individual unless two copies of the recessive allele are present. An allele is a variant form of a gene, and recessive alleles can be masked by dominant alleles. For example, in the case of eye color, the allele for brown eyes is dominant, while the allele for blue eyes is recessive. A person must inherit the blue eye allele from both parents to have blue eyes. When both parents carry a recessive allele, there is a chance their offspring will express the recessive trait. This is often illustrated using a Punnett square, a tool that predicts the genetic outcomes of a cross between two individuals. If both parents are carriers of a recessive trait, like cystic fibrosis, there is a 25% chance their child will inherit the condition.