Dictionary > Complete dominance

Complete dominance

Complete dominance
n., [kəmˈpliːt ˈdɒmɪnəns]
Definition: when the dominant allele completely masks the effect of the recessive allele in heterozygous condition

Complete Dominance Definition

Genetics is the study of how traits are inherited by organisms and in what ways these inheritances can differ. Within the study of genetics, many different processes occur that bring out a vast array of traits in living things. These traits are usually determined by the type of genes that are passed on to the offspring by their parents.

Traits can appear in many different forms of a gene. The variant form of a gene that usually determines traits is called an allele. The combination of those two alleles assigns the organism’s genotype. The genotype traits are seen through the phenotype – which displays the genes and their properties physically. Genotypes and phenotypes can give the result of dominant and recessive alleles. Recessive alleles will only be seen in the absence of the dominant alleles. The completely dominant alleles are those that are displayed in the phenotype.

What is complete dominance? To define complete dominance in biology, we must recall what it means to be dominant. Something that is dominant has complete power or control over something else. So a dominant phenotype would be one that results from a dominant gene and a recessive phenotype would only result when the dominant is absent. This is according to the principle of dominance. The law (or the principle) of dominance states that the presence of a dominant allele will always mask the presence of a recessive allele.

Complete dominance is a form of dominance in the heterozygous condition wherein the allele that is regarded as dominant completely masks the effect of the allele that is recessive. For instance, for an individual carrying two alleles that are both dominant (e.g. AA), the trait that they represent will be expressed. But if the individual carries two alleles in a manner that one is dominant and the other one is recessive, (e.g. Aa), the dominant allele will be expressed while the recessive allele will be suppressed. Hence, the heterozygote (Aa) will have the same phenotype as that of the dominant homozygote (AA). This condition is called complete dominance.


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Biology definition:
Complete dominance is a form of dominance wherein the dominant allele completely masks the effect of the recessive allele in heterozygous conditions. A gene (or allele) shows dominance when it suppresses the expression — or dominates the effects — of the recessive gene (or allele). Compare: incomplete dominance, codominance

There are different types of dominance: incomplete dominance, co-dominance and complete dominance. Incomplete dominance occurs when there is a relationship between the two versions of a gene, and neither is dominant over the other so they mutate to form a third phenotype.

For example, a flower may be red when both are RR dominant alleles are passed down or white when both the rr dominant alleles of the same gene are passed down from the same parents. However, when one allele is R and the other is r, the flower will become a mixture of the two colors, since both alleles are dominant; hence creating a pink offspring (Rr). This can be seen in Figure 1 below.

incomplete dominance cross 1
Figure 1: White (rr) and Red (RR) flowers creating pink offspring through incomplete dominance. A test cross between red and white flower-bearing plants produced offspring that blooms with pink flowers. Image Source: Maria Victoria Gonzaga & Ghulam Mujtaba of BiologyOnline.

Codominance occurs when both the alleles are dominant together and so the traits of both the alleles will show up in the phenotype. Using the example of flowers that can have yellow petals (YY) and red petals (RR), if they are co-dominant, when the offspring receives an allele from each flower parent, it will display both red and yellow petals as the phenotype. Codominance is often seen in cases of plants and animal physical appearances. These can include fur types, for instance, when a brown dog and a black dog mate and create puppies that have brown and black patches.

Flower with red and yellow petals
Figure 2 Flower with red and yellow petals.

Finally, complete dominance will occur when one allele is the only genotype seen in the phenotype. The dominant allele completely cancels out the effects of the recessive allele once it is present – heterozygous conditions. Complete dominance is often interchanged with simple dominance. This is because simple dominance happens when a single gene has two versions of itself. These versions are dominant or recessive. So the organism will either receive the dominant version of the gene when the dominant gene is present and the recessive phenotype in the absence of the dominant genetic trait.

Complete Dominance Examples

Here are the different examples of complete dominance

  • Eye color
  • Dwarfism
  • Mendel’s Peas

Eye color

A typical example of complete and is very frequently seen is eye color. In humans, eye color is influence by numerous genes, and these all code for the six main eye colors: amber, blue, brown, green, grey, and hazel. These colors are based on the amount of melanin that is present in the iris. Brown, is the most common eye color in the world with between 55 and 79% of the world’s population having brown eyes. Thus, the brown color is regarded as the dominant trait. People with brown eyes have a great amount of melanin in their eyes.

Human perception in action
Figure 3: Brown eye color is regarded as the dominant trait in humans.

Blue eyes are known for being one of the most mesmerizing eye colors in the world. However, blue is quite rare. Only about 8-10% of the world’s population has blue eyes with the majority concentrated in Europe. This makes it the second to least most popular eye color in the world. Blue-eyed persons have the least amount of melanin present in their eyes. So are blue eyes recessive or dominant?

As stated previously, eye color is stated by over 150 genes. So though a child’s eye color can typically be determined by looking at their parent’s eyes, sometimes their offspring can have an unpredictable eye color. The gey gene, one of the genes coding for eye color, has a green-eye allele and a blue-eye allele. The bey2 gene which also codes for eye color carries has an allele for brown eyes and another for blue eyes. However, the brown-eye allele is always dominant over the blue and green-eyed alleles. This makes blue and green eyes a recessive trait. This means that for a child to obtain blue or green eyes, they must receive both of the blue or green-eyed alleles.

However, this doesn’t necessarily mean that both their parents must be blue-eyed people. The parents could be brown-eyed but each carries the recessive gene and so their offspring has a one in four chance of being born with blue eyes.

 

Dwarfism

There are many types of dwarfism that occur in humans. A person is considered a dwarf or a “little person” when their height is genetically or medically influenced and they are shorter than four feet, ten inches. Dwarfs can range from 2’8” to 4”8’ but are typically about 4”0’. The most common type of dwarfism is caused by the FGFR3 gene. Overacting and mutating of this gene can lead to numerous medical conditions including cancer and if a child inherits the gene from both parents it can be lethal. Only one copy of the mutated version of the gene is needed to cause dwarfism in a child. For this reason, dwarfism is actual an example of complete dominance because once one of the FGFR3 mutated genes is present, then the child will be a dwarf.

This is a form of dominance inheritance. Autosomal dominant inheritance occurs when a genetic trait or disease is passed down from a single parent to their child. “Autosomal” refers to the position of the gene, meaning it is on a non-sexual and numbered chromosome. “Dominance” referring to the fact that only one copy of the gene is needed to cause illness or disease. This is much different from recessive medical conditions and diseases since both copies of the mutated allele are needed to cause the disease or condition.

For instance, if a parent is a dwarf and the other parent is of “normal” height, their offspring will either be dwarfs as well or regular height people. This is because the dwarf parent can give their regular allele to the offspring or their dwarf-encoding allele to their offspring. The children however would have a higher chance of being regular height than a dwarf.

If two dwarf persons (Aa) decide to have children, their offspring will still have a chance of being regular height since they both could pass on the regular gene to their offspring. However, their offspring have a 50% chance of being dwarfs and also a 25% chance of not surviving since any offspring that receives two copies of the dwarf gene will pass on.

dwarfism - achondroplasia Punnett Square phenotypic and genotypic ratios
Figure 4: Test cross of two individuals with achondroplasia  (the common form of dwarfism) – the offspring that receives two dominant genes (AA) for achondroplasia will not survive at pregnancy as AA is lethal. There is one out of four (1/4) chances (or 25% probability) that the offspring might inherit the lethal gene. Similarly, there is also one out of four chances (or 25% probability) that the offspring would have no achondroplasia. Conversely, there are two out of 4 chances (or 50% probability) that their children would exhibit achondroplasia. Image Source: Maria Victoria Gonzaga of Biology Online.

Mendel’s peas

One of the first scientists to study genes in-depth in terms of dominant versus recessive was Gregor Mendel. He is best known for his work with peas where he discovered that some of the features or traits of the peas and pea plant were more common or dominant than others. The others were seen much less frequently and would only occur if particular plants were mixed – hence being recessive traits.

Pea plant traits
Figure 5: Mendel’s Peas and their different dominant versus recessive traits. Credit: Charles Molnar and Jane Gair, CC BY-SA 4.0.

As seen in the Figure above, the dominant traits for the pea plants would be smooth skin, yellow peas, purple flowers, inflated seed pods, green pod color, the axial position of flowers, and tall stems. Mendel’s work helped answer many questions on dominance versus recessive and other allele and gene questions in genetics. This work also began further studies and discoveries in human genetics and explained why many genes are dominant and recessive today.


Learn more about the other types of dominance. Come and join us here: Incomplete dominance vs. codominance


 

Further Reading

References

  • Bank, E. (2017). What is the Dominant Phenotype?. Sciencing. https://sciencing.com/dominant-phenotype-17703.html
  • Khan Academy. (2021). Co-dominance and Incomplete Dominance. Khan Academy. https://www.khanacademy.org/science/in-in-class-12-biology-india/xc09ed98f7a9e671b:in-in-principles-of-inheritance-and-variation/xc09ed98f7a9e671b:in-in-non-mendelian-genetics/v/co-dominance-and-incomplete-dominance#:~:text=In%20complete%20dominance%2C%20only%20one,is%20seen%20in%20the%20phenotype.
  • McManus, K. (2014). Diseases. The Tech Interactive. https://genetics.thetech.org/ask-a-geneticist/inheriting-dwarfism
  • Lumen Learning. (2021). Laws of Inheritance. Lumen Candela. https://courses.lumenlearning.com/boundless-biology/chapter/laws-of-inheritance/#:~:text=Mendel’s%20law%20of%20dominance%20states,allele%20will%20be%20expressed%20exclusively.
  • Robertson, S. (2019). Genetics of Eye Colour. News-Medical Life Science. https://www.news-medical.net/health/Genetics-of-Eye-Color.aspx
  • Scitable. (2021). Alleles. Scitable by Nature Education. https://www.nature.com/scitable/definition/allele-48/#:~:text=An%20allele%20is%20a%20variant,genetic%20locus%2C%20on%20a%20chromosome.&text=Alleles%20contribute%20to%20the%20organism’s,alleles%20are%20dominant%20or%20recessive.
  • Winchester, A. (2020, May 15). Genetics. Encyclopedia Britannica. https://www.britannica.com/science/genetics

 

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