Dominance is the cornerstone of the theory of Mendelian inheritance. In essence, dominance is used to describe the relationship between two alleles of one gene in which one allele is dominant while the other is recessive. The dominant allele's contribution basically overwrites the recessive allele's effects.
This dominance creates an observable trait. While the phenotype, or set of observable traits, is certainly affected by dominance, the actual inheritance of alleles is not. Since offspring inherit genes that may be recessive and not show up, the next generation of offspring might still show that trait, depending on the alleles of the other parent. Because of this, there are three types of dominance.
The dominant allele in a pair of heterozygous alleles is expressed fully while the recessive allele remains completely hidden. It is still present and can still be passed on to offspring, but is not otherwise expressed in the individual.
An example of complete dominance is when a violet inherits a purple color allele from one parent and a white allele from the other. Each offspring with the purple allele will produce purple flowers because this allele has complete dominance. The offspring flowers will be purple regardless of whether they are homozygous with two purple alleles or heterozygous with one purple and one white. Only if the offspring is homozygous with two white alleles will a white flower occur.
Co-dominance occurs when the phenotypes of both homozygous parents are expressed at the same time. In other words, the offspring inherits different alleles from each parent, but expresses both. This happens when both alleles are either dominant or recessive alleles. The gene for blood types in humans is a good example. When an individual with type A blood mates with an individual with type B blood, the offspring can have type AB blood.
Incomplete Dominance occurs when one allele for a specific trait is not completely expressed over the other. This results in a third phenotype, in which the expressed physical trait is a combination of both alleles. Unlike complete dominance, one allele does not mask the other. Incomplete dominance occurs in the polygenic inheritance of traits like eye and skin color.
For example, let's look at a red flower and a white flower. Neither color allele is fully dominant. When a cross is made between homozygous white and homozygous red flowers, it can result in a variety of pink shades.
Different individuals can have different versions of the same gene. These different versions are called alleles, and when two individuals reproduce, their offspring inherits a mix of alleles from both parents. If the parent alleles are different, one of three things will happen. If one allele is dominant and the other recessive, then the dominant allele will determine the observable trait. This is called complete dominance, and can often be seen in the flower color of many plants. If both alleles are dominant or recessive, then a blending of traits can occur. This happens often with blood type in humans, and is called co-dominance. Incomplete dominance occurs when neither gene is fully expressed over the other. Examples of co-dominant traits are human eye and skin color.