Dominance is one of the cornerstone concepts that comprise the theory of Mendelian inheritance and scientists’ understanding of genetics. In essence, dominance is used to describe the relationship between the alleles that make up a gene, in which one allele’s phenotype is dominant while another is recessive. This dominance means that the dominant allele’s contribution essentially overwrites the recessive allele’s effects. There are three key types of dominance associated with classical genetics.

• Complete dominance: This is where the dominant allele in a pair of heterozygous alleles is expressed fully, while the recessive allele is masked completely.
• Co-dominance: Co-dominance occurs when both alleles in a heterozygous pair are mutually dominant or recessive, meaning that both are expressed together.
• Incomplete dominance: Incomplete dominance is also known as partial dominance, and this will be discussed in more detail down below.

Incomplete Dominance Overview

Incomplete dominance in genes occurs when the one of the alleles in a heterozygous pair is unable to fully express its features. This can often leave the pair indeterminate, with elements of each allele being expressed in a subdued status. Incomplete dominance happens when each genotype has its own individual phenotype, which causes neither allele to truly dominate the other, either because neither are dominant or because the dominant allele fails to dominate the recessive allele fully.

Incomplete Dominance Examples

Incomplete dominance is found throughout the natural world, and is prevalent in flora and fauna alike. It has been observed in plants, animals and even humans. Here are some key incomplete dominance examples in these different categories.

1. Plants

Occasionally, flowers such as snapdragons can inherit strange pigmentation during cross-pollination. Pink snapdragons come about when a red and a white snapdragon are cross-pollinated and if neither the red allele nor the white allele is dominant. The red pigmentation found in red snapdragons is halved when cross-pollinated, resulting in a pink hue.

• Similarly, pink tulips occur when red and white tulips are crossed with one another, as neither of these colors are dominant over the other, or if the protein is simple inactive in one of the pair.
• Carnations and roses are subject to the same rules of pigmentation as in both snapdragons and tulips. With a combination of red and white, they can result in a pink plant as opposed to a red or white dominance.
• Japanese Four o’ Clock flowers follow exactly the same pattern as the above examples. The cross-pollination of a red and a white Four o’ Clock flower will result in flowers that are pink in hue.

2. Animals

The Andalusian fowl is one of the most famous examples of incomplete dominance in animals. A white cock and a black hen will most commonly produce offspring with blue splashes on its feathers, due to the fact that in both parents the inheritance of blue alleles is fifty percent.

• Certain breeds of rabbits, such as the long-furred Angora and the short-furred Rex, are good incomplete dominance examples. When these two are bred together, they produce offspring which have fur that falls somewhere in between the length of that of the two parents.
• Red bulls being bred with white cows will often result in cows with a brown (or faded) coat and spots of white (or vice versa) due to neither the white nor the brown pigmentation being dominant over one another.
• Blue and red birds will often produce offspring with a mixture of both blue and red feathers because of the lack of a dominant color. In some other cases, blue and red birds can produce offspring with purple feathers for the same reason, if one of the alleles has a non-functioning protein.
• Tail length in dogs is directly impacted by incomplete dominance, with several short-tailed and long-tailed dogs producing a dog whose tail length sits somewhere in between.
• A highly spotted dog or cat that mates with a non-spotted member of the same species will most often produce offspring that has some spots, but not as many as the spotted parent.

3. Humans

The hair on a baby’s head is likely to be wavy or gently curled when a straight-haired person and a curly-haired person have a child. This is due to both of these genes being dominant, therefore, neither dominating the other.

• When one parent carries normal hemoglobin and the other with abnormal hemoglobin, the two will have a baby with a disease known as sickle cell disease.
• Tay-Sachs disease, similarly, happens when only half of the necessary antibodies are present during a child’s formation, which makes the child has weak immune system, leaving him or her open to the disease.
• Physical features such as skin color are famous incomplete dominance examples. A dark-skinned person and a light-skinned person will give birth to a child which has skin pigmentation somewhere in between that of its parents.
• Height is also directly affected by dominance, as a tall person and a short person will often produce a person of medium stature if neither the tall nor short allele is dominant over the other.
• Voice pitch is quite often decided by incompletely dominant alleles. If one parent’s voice is notably high and the other’s notably low, this can often produce a person whose voice sits somewhere in between. If neither is dominant or both are dominant, the result will be the same.
• Hand size can be directly influenced by the lack of a completely dominant or co-dominant pair of alleles. Should one parent have large hands and the other with small hands, the result will often be an offspring with medium-sized hands.
• Other physical features such as the prominence of lip protrusions are directly caused by incomplete dominance. Having parents with small and large lip protrusions will often cause the offspring to have a medium lip protrusion if neither of these alleles is dominant over the other.