Dominant Gene: Definition, Dominance, Codominance, Overdrive and Multiple Series

It is a piece of DNA information found on chromosomes.

Dominant genes are the result of dominant alleles on chromosomes. When an allele is prevalent, it is expressed in the phenotype of a recessive allele. The phenotype is the chromosome that is visible in the body.

Therefore, a person with a dominant allele for brown hair and a recessive allele for red hair (this combination is identified in genetic notation as “Br”) will have brown hair.

A child inherits only half of the DNA information from each parent, and in this combination, it usually shows the dominant genes it received. If they inherited only recessive genes for some trait, that is the trait shown.

This is why siblings can look very different and why a person with two brown-eyed parents can be born with blue eyes.

The brown eye gene is dominant, and the blue eye gene is recessive; if the child inherits a recessive blue eye gene from each parent, they will have blue eyes.


Mendel believed that genes behave like atoms that make up a pure substance. Genes can be combined in various ways, but they always maintain distinct identities.


For example, in a cross between two purebred parents with different characteristics such as seed color, the hybrid offspring would have both alternate genes for green and yellow seed color.

Mendel proposed that although both alternate genes are present, there is no color mixing because the alternate gene for yellow is “dominant” over the alternate gene for green. The dominant trait is seen whenever a single copy of its gene is inherited.

When he crossed the hybrid offspring, the green seeds reappeared in the next generation.

Mendel reasoned that the “recessive” green trait is shown only when one copy of the recessive gene is inherited from each parent.

One of Gregor Mendel’s most significant contributions to the study of heredity was the concept of domination.

Mendel observed that a heterozygous offspring could display the same phenotype as the homozygous parent. Thus he concluded that some traits dominated other inherited traits.

However, the relationship of genotype to phenotype is seldom as simple as Mendel’s dominant and recessive patterns.

As the study of heredity expanded beyond the seven traits, Mendel initially examined organisms other than pea plants; biologists began to notice a variety of relationships between alleles.

These allelic interactions were not exclusively recessive or dominant, and they greatly enriched the understanding of how genotype leads to phenotype.

Mendel’s early work with pea plants provided the fundamental knowledge for genetics. Still, Mendel’s simple example of two alleles, one dominant and one recessive, for a given gene is a rarity.

Dominance and recessivity are not allelic properties.

Instead, they are effects that can only be measured relative to the effects of other alleles in the exact location.

Furthermore, dominance can change according to the level of organization of the phenotype—variations in dominance highlight the complexity of understanding genetic influences on phenotypes.

Complete versus partial dominance

Dominance affects the phenotype derived from an organism’s genes but does not affect how these genes are inherited.

Complete dominance occurs when the heterozygous phenotype is indistinguishable from that of the homozygous parent.

However, sometimes the heterozygous displays a phenotype that is an intermediary between the phenotypes of both homozygous parents (one of which is homozygous dominant, and the other is homozygous recessive).

This intermediate phenotype is a demonstration of partial or incomplete dominance. When partial power occurs, a range of phenotypes is generally observed among the offspring.

Many pink-shaded phenotypes are obtained when homozygous red flowers (A1A1) are crossed with homozygous white (A2A2).

However, partial dominance is not the same as mixing inheritance; when two F 1 pink flowers are crossed, red and white flowers are found between the progeny.

In other words, nothing is different about how these alleles are inherited; the only difference is how the alleles determine the phenotype when combined.


Unlike partial dominance, codominance occurs when the phenotypes of both parents are simultaneously expressed in the same offspring organism.

In fact, “codominance” is the specific term for a system in which one allele from each homozygous parent is combined in the offspring, and the offspring simultaneously demonstrate both phenotypes.

An example of codominance occurs in the human ABO blood group system.

Many blood proteins contribute to the individual’s blood type, and the ABO protein system defines explicitly which types of blood you can receive in a transfusion.


In some cases, the offspring may demonstrate a phenotype outside the range defined by both parents.

In particular, the phenomenon known as over-dominance occurs when a heterozygote has a more extreme phenotype than either parent.

A well-known example of over-dominance occurs in alleles that code for sickle cell disease.

Sickle cell anemia is a debilitating disease of red blood cells. The deletion of a single amino acid causes a change in the conformation of hemoglobin in a person so that the individual’s red blood cells are elongated and slightly curved, adopting the sickle shape.

This change in shape makes these red blood cells less efficient at carrying oxygen through the bloodstream.

The altered form of hemoglobin that causes sickle cell disease is inherited as a codominant trait.

Specifically, heterozygous (Ss) individuals express normal and sickle hemoglobin, thus having a mixture of standard and sickle red blood cells.

In most situations, individuals who are heterozygous for sickle cell anemia are phenotypically normal.

In these circumstances, sickle cell disease is a recessive trait.

However, homozygous individuals for the sickle cell (ss) allele can present with serious health problems. In severe cases, this condition can be fatal.

Impaired hemoglobin production may benefit the inhabitants of countries affected by falciparum malaria, a highly deadly parasitic disease.

Sickle blood cells “collapse” around the parasites and filter them out of the blood.

Therefore, people who carry the sickle cell allele are more likely to recover from malaria infection.

The Ss genotype has an advantage over the SS genotype in fighting malaria. It results in resistance to malaria, and the ss genotype, because it does not cause sickle cell crises.

Multiple allele series and dominance

Allelic dominance always depends on the relative influence of each allele for a specific phenotype under certain environmental conditions.

For example, in the pea plant (Pisum sativum), the time of flowering follows a monohybrid (single-gene) inheritance pattern in specific genetic backgrounds.

While there is some variation in the exact time of flowering within plants of the same genotype, specific alleles at this locus (Lf) can exert temporal control of flowering at different origins.

Dominant genes

Most physical traits are determined by genes in the genetic material from the parents. Some genes are dominant, others recessive.

Dominant genes are the most prominent if they are present. Dominant human genes focus on common dominant traits in the majority of the population, such as:


Being right-handed is the norm.

About 70 to 90 percent of people are right-handed because right-handed is a dominant gene.

Dark hair

The vast majority of people in the world have black or brown hair.

These colors are genetically dominant for all other hair tones; 98% of people have dark hair.

Ability to test Phenylthiocarbamide

Phenylthiocarbamide is an organic sulfur compound that is rarely found outside of laboratories.

Various studies conducted worldwide show that the ability to test Phenylthiocarbamide is a dominant trait.

While this sounds like a relatively harmless genetic trait, the ability to perceive Phenylthiocarbamide correlates with the ability to taste bitter things in general.

Many bitter things are also toxic, which means that this ability may be a dominant trait because those without the feature died eating toxins that they could not taste.

Brown eyes

More than 55% of the world’s population has brown eyes. That means brown eyes dominate the other five colors combined, hazel, blue, green, silver, and amber.

Free ear lobe

If you look at the earlobes, you can see that some are attached directly to the side of the head, while others are free.

These free earlobes come from dominant genes and are much more common than their attached counterparts.

While there is no definitive global study, it is estimated that more than two-thirds of the human population have free earlobes.

Long eyelashes

Long lashes are also a dominant trait when it comes to lashes.

Studies show that long eyelashes are definitely in the majority in many races and ethnicities, including Caucasians and Japanese.

Wide eyebrows

Thick eyebrows are a dominant trait.

Left thumb crossover

When putting the hands together as if praying, the thumb that remains at the top is left, showing the dominant gene.

Studies have found that more than half of the world’s population exhibits this trait, a random accident of genetics.

round face

Faces are classified as square or round, and subcategories are classified into various types.

Round is the genetically dominant one of the two main categories of facial shapes.

Although square faces appear more in certain breeds, round faces are dominant worldwide.


Dimples are irregular and dominant.

Due to this irregularity, geneticists assume that dimples come from more than one gene.

Wide lips

Wide lips are a dominant feature.

Certain breeds, those from hot climates, display this trait more commonly than others. Thin lips and small nostrils help the body retain heat, making them genetically advantageous in cold temperatures.

The widow’s peak on the hairline

Widow’s peak is a V-shaped hairline in the middle of the forehead and a genetically dominant trait.