Final answer:
Alleles are alternative versions of a gene found at specific locations on chromosomes, which determine an organism's traits. Individuals can be homozygous or heterozygous depending on whether their alleles are identical or different, and this genetic variation contributes to the diversity of phenotypes within a population.
Step-by-step explanation:
Alleles are alternative versions of the same gene that determine specific traits in an organism. Diploid organisms like humans have chromosomes in pairs, and genes are found at specific locations on these chromosomes. An individual has two alleles for each gene, one on each chromosome of the pair. If both alleles are identical, the individual is said to be homozygous for that trait. If the alleles are different, the individual is heterozygous. This genetic variation can significantly impact an individual's phenotype, which is the observable characteristics.
Moreover, while Mendelian genetics were initially thought to involve only two alleles (one dominant and one recessive), we now understand that multiple alleles can exist at the population level. These multiple alleles allow for a great diversity of genotypes and phenotypes within a population. The most common phenotype or genotype is typically referred to as the wild type, with all other forms considered variants.
Understanding that different versions of genes, called alleles, are responsible for the genetic variation among individuals helps explain the wide range of traits and characteristics seen in populations.