Blue Eye Genetics
There are two genes located at different loci. One gene, called B for simplicity, confers brown eye color, and it is dominant over the b allele (recessive) which gives rise to the blue eye color. The other gene, named G, also has two alleles: G (dominant for green) and g (recessive) for lighter greenish colored eyes.
Considering these two genes (B and G), and the fact that people have two copies of a gene (one from mom and one from dad) and that B is dominant over b, G dominant over g, and that the gene B is dominant over G, the genotype for true blue eyes must be bbgg.
To evaluate the possibilities of eye colors based on this simple two gene model we can build up a matrix (called Punnett Square) for a cross between parents who have the dominant and recessive alleles of both genes. It is also assumed that these two genes segregate independently, that is, that each one behaves independently of the other (this assumption is valid if they are located on different chromosomes for example). This model gives rise to sixteen possibilities of alleles at the two loci combined and segregated independently.
According to this model the presence of a B allele will give brown eyes because of its dominance. There are 12 possible genotypes for brown eyes (see Punnett square), 2 for green eyes (bbGG, bbGg), and one for blue eyes (bbgg).