Mendel discovered that alleles for different traits can be dominant or recessive. An exception is the incomplete dominance, where neither one of the two alleles is completely dominant over the other. It means that the heterozygous genotype will not appear as the phenotype of homozygous genotype, recessive nor dominant.

Gene expression can be affected by environmental factors, which can alter the phenotype of an organism.

Some physical characteristics, such as the color of the human skin or eyes, are controlled by at least two genes. These traits are known as polygenic traits. The number of genes that determine the color of the iris is unknown, but the most significant of them are found in Chromosome 15 and they are named OCA2 and HERC2. These genes are associated with the production of melanin.

The color of flamingos will depend on the amount of carotenoids they ingest through food. They eat algae and crustaceans, which contain these substances. Carotenoids are metabolized in the liver into the pink and orange pigments that bind with the fat molecules in the feathers, bill, and legs. If we would observe captive flamingos, we would notice that the young ones have gray feathers. If they eat food that is poor in carotenoids, the color of their feathers will be pale. If, however, they eat food reach in carotenoids, such as algae or even supplements, their color will become deeply pink.

Mendel’s discovered that alleles for different traits can be dominant or recessive. An exception is an incomplete dominance, where neither one of the two alleles is completely dominant over the other. It means that the heterozygous genotype will not appear as the phenotype of homozygous genotype, recessive nor dominant. The petal color in roses is an example of incomplete dominance. Dominant homozygous genotype will give red, while the recessive homozygous genotype will give white petals. Heterozygous genotype will produce a pink color of petals. If the gametes, which are blue colored, are heterozygous, the Punnett square will look like this:

begin{center}
begin{tabular}{ c c c}
& {color{Blue}R} & {color{Blue}r}\
{color{Blue}R} & RR & Rr\
{color{Blue}r} & Rr & rr\
end{tabular}
end{center}