WHAT IS RECESSIVITY AND DOMINANCE? - BIOLOGY - 2025

The term recessivity is used in genetics to describe the relationship between two alleles of the same gene. When we refer to an allele whose effect is masked by another, we say that the first is recessive.

The term dominance is used to describe the same relationship between alleles of a gene, although in the opposite sense. In this case, when referring to the allele whose effect masks the other, we say that it is dominant.

Figure 1. Gregorio Mendel, considered the father of Genetics. Source: By Bateson, William (Mendel's Principles of Heredity: A Defense), via Wikimedia Commons

As can be seen, both terms are deeply related and are usually defined by opposition. That is, when it is said that one allele is dominant with respect to another, it is also saying that the latter is recessive with respect to the first.

These terms were coined by Gregor Mendel in 1865, from his experiments with the common pea, Pisum sativum.

Recessivity and dominance in multi-allelic genes

Multi-allelic genes

Dominance and recessivity relationships are easy to define for a gene with only two alleles, however; these relationships can be complicated in the case of multi-allelic genes.

For example, in the relationship between four alleles of the same gene, it could happen that one of them is dominant with respect to another; recessive with respect to a third, and codominant with respect to a fourth.

Genetic polymorphism

Genetic polymorphism is called the phenomenon of a gene presenting multiple alleles in a population.

Origin of the terms "dominant and recessive"

Gregory Mendel's experiments with peas

The terms dominant and recessive were introduced by Mendel to refer to the results he obtained in his crossbreeding experiments with the pea Pisum sativum. He introduced these terms, studying the trait: "flower color."

Pure lines

Pure lines are populations that produce homogeneous offspring, either by self-pollination or cross-fertilization.

In his first experiments, Mendel used pure lines that he had maintained and tested for over 2 years to ensure their purity.

In these experiments he used as parental generation, pure lines of plants with purple flowers, crossed with pollen of plants with white flowers.

Mendel's first results

Regardless of the type of crossing (even if it pollinated white flowers with pollen from purple flowers), the first filial generation (F ₁) had only purple flowers.

In this F _{2 he} observed constant proportions of approximately 3 purple flowers for each white flower (3: 1 ratio).

Mendel repeated this type of experiment, studying other characters such as: the color and texture of the seeds; the shape and color of the pods; the arrangement of the flowers and the size of the plants. In all cases, he achieved the same result regardless of the character tested.

Figure 2. Characters selected by Gregorio Mendel in his experiments with peas (Pisum sativum). Source: (By Mariana Ruiz LadyofHats (Spanish Translation El Ágora), via Wikimedia Commons)

Then Mendel allowed the self-pollination of F ₁, obtaining a second filial generation (F ₂), in which the white color reappeared in some flowers.

Later experiments

Later Mendel understood that the F ₁ plants, despite presenting a certain character (such as the purple color of the flowers), maintained the potential to produce offspring with the other character (white color of the flowers).

The terms dominant and recessive were then used by Mendel to describe this situation. Ie called dominant phenotype appearing in the F ₁ and recessive to the other.

Mendel's Laws

Finally, this scientist's findings were summarized in what is now known as Mendel's Laws.

These explained the operation of various aspects of heredity, laying the foundations of Genetics.

Genes, gene pair and segregation

Genes

The experiments carried out by Mendel allowed him to conclude that the determinants of inheritance have a particulate nature (of a discrete nature).

We call these determinants of inheritance today genes (although Mendel did not use this term).

Gene pair

Mendel also inferred that the different forms of a gene (alleles), responsible for the alternative phenotypes observed, are found in duplicate in the cells of an individual. This unit is called today: gene pair.

Today we know, thanks to this scientist, that dominance and / or recessivity are ultimately determined by the alleles of the gene pair. We can then refer to the dominant or recessive allele as the determinants of said dominance or recessivity.

Segregation

The alleles of the gene pair are secreted in the seminal cells during meiosis and are reassembled in a new individual (in the zygote), giving rise to a new gene pair.

Nomenclature

Notation

Mendel used uppercase letters to represent the dominant member of the gene pair, and lowercase for the recessive.

Alleles of a gene pair are assigned the same letter to indicate that they are forms of a gene.

Homozygous and Heterozygous

For example, if we refer to the pure-lined character "pod color" from Pisum sativum, yellow is represented as A / A, and green is represented as a / a. Individuals who are carriers of these gene pairs are called homozygous.

Carriers of a gene pair of the A / a form (which appear yellow) are called heterozygotes.

The yellow color of the pods is the phenotypic expression of both a homozygous A / A gene pair and a heterozygous A / a gene pair. While the green color is an expression only of the homozygous a / a pair.

Figure 3. Mendel's model representing self-fertilization of a Heterozygous individual. With modification of: (By Pbroks13, from Wikimedia Commons)

The dominance of the "pod color" character is the product of the effect of one of the alleles of the gene pair, since plants with yellow pods can be homozygous or heterozygous.

Dominance and recessivity at the molecular level

Genes and allelic pairs

Thanks to modern molecular biology techniques, we now know that the gene is a nucleotide sequence in DNA. A gene pair corresponds to two nucleotide sequences in DNA.

In general, the different alleles of a gene are extremely similar in their nucleotide sequence, differing only by a few nucleotides.

For this reason, the different alleles are actually different versions of the same gene, and may have arisen from a specific mutation.

Alleles and proteins

The DNA sequences that make up a gene encode proteins that fulfill a specific function in the cell. This function is related to a phenotypic character of the individual.

Example of dominance and recessivity at the molecular level

Take as an example, the case of the gene that controls the color of the pod in the pea, which has two alleles:

• the dominant allele (A) that determines a functional protein and,
• the recessive allele (a) that determines a dysfunctional protein.

Dominance

A dominant homozygous (A / A) individual expresses the functional protein and, therefore, will present the yellow sheath color.

In the case of the heterozygous individual (A / a), the amount of protein produced by the dominant allele is sufficient to generate the yellow color.

Recessivity

The homozygous recessive individual (a / a) expresses only dysfunctional protein and, therefore, will present green pods.

Examples in humans

As mentioned above, the terms dominance and recessivity are related and are defined by opposition. Therefore, if a trait X is dominant with respect to another Z, then Z is recessive with respect to X.

For example, it is known that the trait "curly hair" is dominant with respect to "straight hair", therefore, the latter is recessive with respect to the former.

Dominant physical traits

• dark hair is dominant over light,
• long eyelashes are dominant over short ones,
• the "rolling" tongue is dominant over the "non-rolling" tongue,
• ears with lobes are dominant over ears without lobes,
• the Rh + blood factor is dominant over Rh-.

References

1. Bateson, W., and Mendel, G. (2009). Mendel's Principles of Heredity: A Defense, with a Translation of Mendel's Original Papers on Hybridization (Cambridge Library Collection - Darwin, Evolution and Genetics). Cambridge: Cambridge University Press. doi: 10.1017 / CBO9780511694462
2. Fisher, RA (1936). Has Mendel's work been rediscovered? Annals of Science. 1 (2): 115-37.doi: 10.1080 / 00033793600200111.
3. Hartwell, LH et al. (2018). GENETICS: FROM GENES TO GENOMES, Sixth Edition, MacGraw-Hill Education. pp. 849.
4. Moore, R. (2001). The "Rediscovery" of Mendel's Work. 27 (2): 13–24.
5. Novo-Villaverde, FJ (2008). Human Genetics: Concepts, mechanisms and applications of Genetics in the field of Biomedicine. Pearson Education, SA pp. 289.
6. Nussbaum, RL et al. (2008). Genetics in Medicine. 7th Ed. Saunders, pp. 578.
7. Radick, G. (2015). Beyond the "Mendel-Fisher controversy." Science, 350 (6257), 159-160. doi: 10.1126 / science.aab3846