WHAT IS COMPLETE DOMINANCE? - BIOLOGY - 2025

The complete dominance refers to the unalterable manifestation of a certain character for an allele that is always expressed above others. In it, the presence of the dominant allele masks the manifestation of any other allele (recessive).

Complete dominance is the simplest form of allelic interaction in traits determined by a single gene. The dominant allele generally codes for a functional product, while the mutant recessive is not expressed or expresses a non-functional product.

There are conditions and factors, however, that must be taken into account when defining the complete dominance of one allele over others. At the level of individuals, for example, the character may or may not be expressive.

That is, the manifestation of character can be predictable given the dominant nature of the allele under study. But the mode of expression of the character may not always be the same.

In polydactyly, for example, which is a dominant trait, the dominant manifestation of character is the possession of supernumerary fingers. However, that extra toe does not always appear on the same hand or foot.

In each different individual the expression of character may vary. At the population level, on the other hand, we stumble upon the phenomenon of penetrance. It is clearer to see complete dominance in genes with complete penetrance than in those without.

It is said that a gene has complete penetrance when in a population the individuals that have a certain genotype will always manifest it with the same phenotype.

Finally, there are genes whose phenotypic manifestation will depend on the conditions in which it is expressed. There are, for example, traits modified by the sex of the individual.

In some cases of baldness, it is determined by the presence of a dominant allele in men. In women, for the same condition and same gene, this type of baldness will be manifested only by homozygous recessive women.

The dominant alleles of the same character

A gene can have many alleles. In diploid organisms, of course, an individual will only have two alleles for the same gene from the same locus. But in a population there can be many dominant alleles, as well as many recessive alleles.

In simple conditions, any dominant allele would be the one that allows a character to manifest in all its potentiality. A recessive one, on the other hand, would not allow it.

Therefore, apart from the dominant to recessive relationship that we have already mentioned, it is possible to find relationships between dominant alleles - that do not refer to codominance.

In codominance both alleles in the heterozygous manifest with the same force. In other cases, however, alleles that are dominant over the recessive ones, establish hierarchies of expression among them.

For example, it is possible to find that the A ¹ allele (yellow phenotype, for example) is completely dominant over the a allele (white phenotype). Let's say that the A ² allele is also dominant over the recessive one and determines the appearance of a brown phenotype.

It is then possible to find that only one of the two colors appears in heterozygotes A ¹ A ² and not an intermediate or mixture between them. That is, for example, that A ¹ is dominant over A ², or vice versa.

Dominant multiple alleles and allelic series

When alleles for the same gene are many in a population and lead to variations in the phenotypic expression of the trait, we speak of multiple alleles.

As recessives always recede and do not manifest themselves, there are no hierarchical relationships between them. The hierarchical dominance / expression relationship between different dominant (and recessive) alleles of the same gene is called the allelic series.

This is very common among genes that participate in the manifestation of coat color in animals, or the shape of fruits in plants. In the previous section, for example, if yellow turns out to be dominant over the brown and white phenotypes, the allelic series would be A ¹ > A ² > a.

Heterozygous over-dominance or advantage

We call superdominant or over-dominant, in genetics, the allele that in heterozygous condition allows to overcome the phenotypic manifestation of the dominant and recessive homozygotes.

For example, if the recessive rr constitution in plants allows plants to produce pale pink flowers, the homozygous dominant RR will produce dark pink flowers. Interestingly, the Rr heterozygote, however, will produce red flowers.

It has been proven that at the level of the immune system, individuals who are heterozygous for the genes of the system have better health than those who are homozygous for several of them. This undoubtedly gives an advantage to heterozygotes over those who are not.

The “leaky” phenotype: partially dominant or partially recessive alleles?

The "leaky" phenotype refers to a partial manifestation of a trait, derived from the expression of an allele of incomplete loss of function. In combination with a dominant allele, it behaves recessive; versus a recessive one (loss of function), as dominant.

For example, if we imagine that it is a gene that codes for a monomeric enzyme, the dominant allele E will allow the synthesis of the enzyme in combination EE or Ee.

That is, complete dominance if both genotypes give rise to the same activity and phenotype. Homozygous ee, loss-of-function mutants will not exhibit the activity associated with the enzyme.

There is always the possibility, however, of encountering mutant alleles that allow the synthesis of an enzyme that shows residual or decreased activity.

This could be due, for example, to mutations that affect the active site of the enzyme or its affinity for the substrate. If we call E ^l this allele, the heterozygote EE ^l behave like EE homozygous or heterozygous Ee.

That is, the dominant character trait will manifest. In the combination E ^l e, the “leaky” phenotype will manifest, and not the loss of function. That is, as a dominant allele.

References

1. Brooker, RJ (2017). Genetics: Analysis and Principles. McGraw-Hill Higher Education, New York, NY, USA.
2. Goodenough, UW (1984) Genetics. WB Saunders Co. Ltd, Philadelphia, PA, USA.
3. Griffiths, AJF, Wessler, R., Carroll, SB, Doebley, J. (2015). An Introduction to Genetic Analysis (11 ^th ed.). New York: WH Freeman, New York, NY, USA.
4. Hedrick, PW (2015) Heterozygote advantage: the effect of artificial selection in livestock and pets. Journal of Heredity, 106: 141-54. doi: 10.1093 / jhered / esu070
5. LaFountain, AM, Chen, W., Sun, W., Chen, S., Frank, HA, Ding, B., Yuan, YW (2017) Molecular basis of overdominance at a flower color locus. G3 (Bethesda), 4: 3947-3954.