Alleles are different versions of a gene that determine specific traits in an organism. For example, the gene for flower color in plants can have multiple alleles, such as one for red flowers and another for white flowers. Each individual inherits two alleles for each gene, one from each parent, which can be the same or different. When alleles are the same, the organism is said to be homozygous, while if they are different, it is heterozygous. The combination of alleles influences how traits are expressed, contributing to the diversity seen in species and their characteristics.