Dominant traits are caused by a single allele masking the phenotype of the other allele. Recessive traits require inheriting two copies of the recessive allele to manifest. Gregor Mendel established that purple flower color in peas was dominant over white, as peas needed only one purple allele. When heterozygous, the dominant trait manifests while the recessive allele remains inactive. Examples include capital letters representing dominant traits and lowercase for recessive traits in genetics symbols.