According to Mendel, an organism receives two forms of a gene from each parent. This form received by each parent is called an ALLELE. An allele is the variant form of a gene for a particular trait.
For a diploid organism, two copies of genes (alleles) are acquired by an organism. The combination of these alleles from each parent determines the genotype for that gene.
In Mendel's experiments, he discovered that one allele can mask the expression of another for a trait. He called the allele that masks DOMINANT allele while the allele that is masked RECESSIVE allele. The combination of these alleles, whether two dominant or two recessive called HOMOZYGOUS or one dominant and one recessive called HETEROZYGOUS, determines the phenotype of the organism for that gene.
N.B: There are exceptions to this pattern of inheritance.
