RECESSIVE GENE: ASPECTS, STUDY METHODS, FACTORS, EXAMPLES - BIOLOGY - 2025

A recessive gene is one responsible for defining the "recessive" characteristics of the phenotype of individuals. The phenotype derived from these genes is only observed when individuals have the two recessive alleles in their genotype in a homozygous way.

For an individual to be homozygous, he must possess both alleles for a phenotypic characteristic of the same type. The "alleles" are the alternative forms of a gene, which is the one that encodes each morphological character. These can determine the color of the flowers, the color of the eyes, the propensity to diseases, etc.

The light-eyed character in humans is determined by the expression of a recessive gene (Source: Kamil Saitov, via Wikimedia Commons)

The phenotype is the set of all the characteristics that can be observed, measured and quantified in a living organism. This depends directly on the genotype, since if dominant genes are found together with recessive genes (heterozygous) in the genotype, only the characteristics of the dominant genes will be expressed.

Typically, the characteristics that are expressed from recessive genes are the rarest to observe in a population, for example:

Albinism in animals is a condition that manifests itself only when the genes that determine it are found in homozygous form. That is, when the two alleles present in the genotype are the same and both derive in the absence of color or albinism.

Although there are some variations between animal species and human populations, albinism has been observed to occur with a frequency of 1 in 20,000 individuals.

Historical aspects

The term "recessive" was first used by the monk Gregor Mendel in 1856, when he was studying pea plants. He observed that crossing pea plants that had purple flowers with pea plants with white flowers, only pea plants with purple flowers were obtained.

Both parents of this first generation of crosses (F1) were homozygous, both for the dominant alleles (purple) and for recessive (white), but the result of the crosses gave heterozygous individuals, that is, they had a dominant allele and an allele recessive.

However, the individuals of the first generation (F1) only expressed the purple color of the flowers, derived from the dominant gene, since this masked the white color of the recessive allele.

Mendel determined that the purple phenotype in pea flowers was dominant over the white phenotype, which he called "recessive." The phenotype of white flowers in pea plants only appeared when the plants of the first generation (F1) crossed each other.

Punnett square showing the results of crossing the descendants of the F1 generation that give rise to the F2s (Source: User: Madprime via Wikimedia Commons) When Mendel self-fertilized the first generation (F1) pea plants and obtained the second generation (F2), observed that a quarter of the resulting individuals had white flowers.

Thanks to the work done with pea plants, Mendel is known as the father of modern genetics.

Genetic principles and study methods

Mendel, in his day, did not have the technology to elucidate that the recessive white phenotype in the flowers of pea plants was due to a gene with recessive characteristics. It was not until 1908 that Thomas Morgan demonstrated that the elements of heredity reside in chromosomes.

Chromosomes are a kind of strand made up of chromatin, which, in eukaryotes, is a combination of deoxyribonucleic acid (DNA) and histone proteins. These are located in the nucleus of the cell and are carriers of almost all the information of the cells of living organisms.

In 1909, Wilhelm Johannsen coined the name "gene" to the fundamental unit of heredity and, finally, the English biologist William Bateson put all the information and concepts in order, and started a new science which he called 'genetics'.

Genetics studies how the phenotypic traits of individuals are transmitted from parents to offspring and, usually, classical genetic studies are carried out just as Mendel did: through crosses and analysis of the descendants.

In the crosses, it is evaluated which of the parents transmit in a more “efficient” way the physical characteristics of which they are carriers. This determines whether such physical traits depend on dominant or recessive genes (although it is sometimes a bit more complicated than this).

Factors that influence the expression of a recessive gene

The expression of phenotypic traits from recessive genes depends on the ploidy of the individuals. In the case of humans and most animals, we speak of diploid individuals.

Diploid individuals have only two alleles or different forms of a gene for each character, because of this we can refer to organisms as homozygous or heterozygous. However, there are organisms with three or more different alleles for a gene.

These organisms are classified as polyploid, since they can have three, four or more copies of a gene. For example, many plants are tetraploid, that is, they can have four different copies of the gene that codes for a phenotypic trait.

On many occasions, the recessive genes of populations have harmful effects on their carriers, since, if the dominant genes that are manifested in the phenotype of the individuals had harmful effects, these individuals would be rapidly eradicated by natural selection.

In contrast, as it is common to find harmful effects caused by recessive genes, these are less likely to manifest in the phenotype and less likely to be purged from the population by natural selection. This effect is called the directional domain.

Examples

There are some exceptions in which recessive genes represent an advantage in the phenotype of their carriers, such is the case of sickle cell anemia. This disease causes the red blood cells, instead of presenting a flattened and circular shape, to present a rigid morphology in the shape of a sickle or crescent.

These long, flattened, and pointed blood cells get stuck in capillaries and block normal blood flow to the blood. In addition, they have a lower oxygen transport capacity, so muscle cells and other organs do not have enough oxygen and nutrients and this causes chronic degeneration.

Photograph of a blood smear showing a sickle red blood cell (Source: Paulo Henrique Orlandi Mourao via Wikimedia Commons) This disease is inherited in a recessive manner, that is, only people who have both forms of the gene (homozygous) for sickle form of erythrocytes suffer from the disease; whereas people who have a gene for sickle cells and normal cells (heterozygotes) do not have the disease, but are "carriers".

However, the condition of sickle cell anemia is not as serious in countries where diseases such as malaria predominate, since the morphological characteristics of blood cells prevent them from being “colonized” by intracellular parasites.

References

1. Aidoo, M., Terlouw, DJ, Kolczak, MS, McElroy, PD, ter Kuile, FO, Kariuki, S.,… & Udhayakumar, V. (2002). Protective effects of the sickle cell gene against malaria morbidity and mortality. The Lancet, 359 (9314), 1311-1312.
2. Goodale, HD (1932). Dominant vs. Non-Dominant Genes: In the Multiple Factor Hypothesis of Size Inheritance. Journal of Heredity, 23 (12), 487-497.
3. Haldane, JB (1940). The estimation of recessive gene frequencies by inbreeding. Proceedings: Plant Sciences, 12 (4), 109-114.
4. Patel, RK (2010). Autosomal recessive genetic disorders of cattle breeds Worldwide-a Review. Journal of Livestock Biodiversity, 2 (1).
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