Final answer:
Alleles are different versions of a gene and they segregate during meiosis, according to Mendel's laws. Each parent provides one allele for each gene, and an organism's genotype, which is the combination of alleles, determines its phenotype.
Step-by-step explanation:
According to Mendel's Chromosome theory, alleles are the different versions of a gene that can exist. Gregor Mendel, through his work on pea plants, determined that traits, which are now understood to be determined by genes located on chromosomes, can have multiple forms. These forms, or variations, are what we refer to as alleles. As part of Mendel's genetic laws, the law of segregation states that during the process of meiosis, these alleles are separated so that each gamete carries only one allele for each gene. Mendel's experiments also demonstrate the law of independent assortment, which indicates that alleles for different genes segregate separately from one another during the formation of gametes.
Each individual receives one allele from each parent, thus having two alleles for each gene, and these alleles can be homozygous or heterozygous. The combination of alleles that an individual possesses is known as their genotype, and this determines their phenotype, or observable traits. While Mendel initially proposed the concept of a dominant and a recessive allele, modern genetics has found multiple alleles can exist at the population level beyond the standard two hypothesized by Mendel.