What is complete dominance?

The complete dominance refers to the unalterable manifestation of a character determined by 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 recessive mutant 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 of invariable expressiveness..

That is, the manifestation of the 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 on the same 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 full penetrance than in those that do not.

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.

Article index

• 1 The dominant alleles of the same character
  • 1.1 Dominant multiple alleles and allelic series
• 2 Over-dominance or heterozygous advantage
• 3 The “leaky” phenotype: partially dominant or partially recessive alleles??
• 4 References

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 heterozygote manifest with the same force. In other cases, however, alleles that are dominant over recessive ones, establish hierarchies of expression between them..

For example, it is possible to find that the allele TO¹ (yellow phenotype, for example) is completely dominant over the allele to (white phenotype). Let's say the allele TO^two it is also dominant over the recessive to and determines the appearance of a brown phenotype.

It is then possible to find that in heterozygotes TO¹TO^two only one of the two colors appears and not an intermediate or mixture between them. That is, for example, that TO¹ be dominant over TO^two, or vice versa.

Dominant multiple alleles and allelic series

When in a population the alleles for the same gene are many 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 relationship of dominance / expression between different dominant (and recessive) alleles of the same gene is called allelic series.

This is very common among genes that participate in the manifestation of the color of the coat in animals, or of the shape of the 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 TO¹>TO^two> to.

Over-dominance or heterozygous advantage

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

For example, if the recessive constitution rr in plants it allows them to produce pale pink flowers, the dominant homozygous RR will produce dark pink flowers. Interestingly, the heterozygous Rr, however, it 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 AND 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 mutants ee, loss of function, will not manifest 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 enzyme's active site or its affinity for the substrate. If we call AND^l to this allele, the heterozygote EE^l will behave like the homozygous EE or the heterozygote Ee.

That is, the dominant character trait will manifest. In combination AND^land, The “leaky” phenotype will manifest, and not that of loss of function. That is, as the dominant allele.

References

1. Brooker, R. J. (2017). Genetics: Analysis and Principles. McGraw-Hill Higher Education, New York, NY, USA.
2. Goodenough, U. W. (1984) Genetics. W. B. Saunders Co. Ltd, Philadelphia, PA, USA.
3. Griffiths, A. J. F., Wessler, R., Carroll, S. B., Doebley, J. (2015). An Introduction to Genetic Analysis (11^th ed.). New York: W. H. Freeman, New York, NY, USA.
4. Hedrick, P. W. (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, A. M., Chen, W., Sun, W., Chen, S., Frank, H. A., Ding, B., Yuan, Y. W. (2017) Molecular basis of overdominance at a flower color locus. G3 (Bethesda), 4: 3947-3954.