What makes thin lips a recessive trait?

Thin lips are considered a recessive trait because they typically require two copies of the gene associated with thin lips to be expressed in an individual. If an individual inherits one allele for thin lips and one for fuller lips (the dominant trait), the fuller lip phenotype will prevail. This pattern of inheritance follows Mendelian genetics, where traits are determined by alleles that can be dominant or recessive. Therefore, thin lips manifest only when both alleles are for the recessive trait.