Final answer:
Alleles are slightly different DNA sequences of the same gene that result in genetic variation. These variations can affect different traits, and individuals can be homozygous or heterozygous for a given trait. The dominant and recessive interactions of these alleles shape observable characteristics, known as the phenotype.
Step-by-step explanation:
Alleles are different versions of the same gene, and they can differ from one another by having slightly different DNA sequences. These differences can affect traits such as blood type, hair color, or height. For example, regarding human blood type there are three possible gene sequences (allele types) on the human chromosome: sequence A, sequence B, and sequence O. An individual might inherit one type from each parent, leading to combinations like AA, BB, AB, AO, BO, or OO.
An individual's genotype, which is the specific set of alleles they carry, can be homozygous (the same allele on both chromosomes) or heterozygous (two different alleles on each chromosome). The interaction of these alleles—whether one is dominant or recessive—will determine the individual's phenotype, which is the observable characteristic.