Terminologia genetyczna

Alleles are different forms of the same gene. One is received from each parent resulting in a diploid organism. Two alleles at the same locus for every gene involved.

Each pair is known as the genotype of a particular gene and the physical and observable expression, the phenotype, such as eye color is determined by the inherited combination.

In this case, one of the genes contains two alleles for the potential colors brown and blue. If both forms are identical the genotype is called homozygous.

Whereas if the pair differ the genotype is heterozygous. Interestingly, the same phenotype like brown eyes can be expressed from different genotypes. This is because the dominant allele, brown shown as an upper case letter is expressed over the recessive allele, blue written as a lower case letter. Thus, the only way a recessive allele can be expressed is if both copies are recessive.