GENOTYPE: CHARACTERISTICS, REACTION NORM, DETERMINATION - BIOLOGY - 2025

The genotype is defined as the set of genes (with their alleles) that code for a particular trait or characteristic, which are distinguished from others by a specific function or sequence. However, some authors also define it as the part of the genome that gives rise to the phenotype or as the allelic constitution of an organism.

Although related, the terms genotype and phenotype are not the same thing. In this sense, phenotype is defined as the set of visible characteristics of an organism that are the result of the expression of its genes, and the genotype as the set of genes that gives rise to a particular phenotype.

Genotype and phenotype (Source: National Human Genome Research Institute via Wikimedia Commons) The genotype is only one of the factors involved in the establishment of the phenotype, since the influence of the environment and other epigenetic elements that are not directly related to the nucleotide sequence, they also shape the visible characteristics of individuals.

Thus, two organisms have the same genotype if they share the same gene pools, but the same is not true for two organisms that apparently share the same phenotype, since similar characteristics can be the product of different genes.

It was the Danish botanist Wilhelm Johannsen, in 1909, who introduced the terms genotype and phenotype to science for the first time, in a textbook entitled "The elements of a theory of exact inheritance", which was the product of a series of experiments he carried out crossing pure lines of barley and peas.

His works, probably inspired by those carried out a few years before by Gregorio Mendel, considered the "father of genetics", allowed him to clarify that the genotype of an organism gives rise to the phenotype through different development processes and under the influence of environment.

characteristics

The genotype is not exactly the same as the genome. Here is the distinction between the two concepts:

- "Genome" refers to all the genes that an individual has inherited from their parents and how these are distributed on the chromosomes within the nucleus.

- "Genotype" is the term used to refer, for example, to the set of genes and their variants that give rise to a particular trait, from which an individual is distinguished within a population or a species.

Although it is prone to undergo changes due to mutations throughout the life history of an organism, the genotype is a relatively invariable trait of individuals, since, in theory, the genes that are inherited are the same from conception to death.

In a natural population, the alleles that make up a given genotype have different frequencies of appearance; that is, some appear in populations more than others and this is related, among some things, to distribution, environmental conditions, the presence of other species, etc.

The term "wild genotype" defines the first allelic variant found in nature, but does not necessarily refer to the allele most frequently found within a population; and the term "mutant genotype" is commonly used to define those alleles other than wild type.

To write a genotype, upper and lower case letters are usually used in order to differentiate between the alleles that an individual possesses, whether homozygous or heterozygous. Capital letters are used to define dominant alleles and lowercase letters for recessive ones.

Genotype reaction standard

Individuals inherit genes from their parents, but not the final products that are obtained from their expression, because these depend on many external factors and the history of their development.

In accordance with this and referring only to environmental factors, a genotype can give rise to more than one phenotype. The set of possible "outcomes" of the interaction of a specific genotype with different environments is what scientists have called the "genotype reaction norm".

The norm of reaction of a genotype is, then, a kind of "quantification" or recording of the visible characteristics that are obtained from the interactions of a genotype with certain environments. It can be expressed as graphs or tables that "predict" the possible outcomes.

It is clear, of course, that the reaction norm refers only to a partial genotype, a partial phenotype, and a few environmental factors, since in practice it is very difficult to predict absolutely all interactions and all their outcomes.

How is the genotype determined?

Determining the genotype or "genotyping" an organism or a population of individuals of the same species, provides much valuable information regarding its evolutionary biology, population biology, taxonomy, ecology and genetic diversity.

In microorganisms such as bacteria and yeasts, since they have higher multiplication and mutation rates than most multicellular organisms, determining and knowing the genotype allows controlling the identity of the colonies in the collections, as well as establishing some characteristics of epidemiology, ecology and taxonomy of the same.

To determine the genotype, it is necessary to obtain samples of the organism with which you want to work, and the types of samples needed will depend on each organism. In animals, for example, samples can be taken from different tissues: the tail, ears, feces, hair or blood.

The genotype of an organism can be determined experimentally thanks to the use of some modern techniques, which will depend on the genomic location of the genes to be studied, the budget and time, the ease of use and the degree of performance that is desired.

At present, the techniques used for the genotyping of an organism include, very often, the use and analysis of molecular markers to detect polymorphisms in DNA and other more advanced techniques that involve genome sequencing.

Most used markers

Among the most used markers we find the following:

- RFLPs (restriction fragment length polymorphisms).

- AFLPs (amplified fragment length polymorphisms).

- RAPDs (random amplified polymorphic DNA).

- Microsatellites or SSRs (single sequence repeats).

- ASAPs (primers associated with specific alleles).

- SNPs (single nucleotide polymorphisms).

Techniques that use sequencing and hybridization

And among the techniques that use specific probe sequencing and hybridization are:

- Sequencing by the Sanger method.

- High performance genotyping.

- Illumina's “GoldenGate” essay.

- Genotyping by sequencing (GBS).

- TaqMan Assay.

- Next generation sequencing.

- Microarrays.

- Whole genome sequencing.

References

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