8 junio, 2024

Genotypic variations: characteristics, types, examples

Genotypic variations are those differences at the level of genetic information (genotype) that can be found among individuals in a population. These differences are often the result of mutations or recombination events during cell division.

While the phenotype represents the set of visible characteristics of an organism (arising from the interaction of the genotype with the environment where it develops), the genotype constitutes the underlying genetic compendium that gives rise to said characteristics.

Therefore, we say that each trait or physical characteristic of a living being is determined by the expression of its genes, as well as by the interaction between them. Understand as a trait, for example, the color of the eyes, the shape of the hair, the weight, the height, the metabolism, the personality, etc.

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What is the genotype?

The genotype represents the complete set of genes of an individual. Genes are the hereditary units of information and can be found in different alternative forms in the same population. Alternative forms of a gene are called alleles.

Diploid (2n) organisms that reproduce sexually have two alleles or forms of each of their genes, one randomly inherited from their mother and one randomly inherited from their father; which is why the offspring is genetically different from the parents and genetically different from each other.

Allelic variations are often the result of changes in the DNA sequence due to mutations, the combination or mixing of genes between homologous chromosomes (recombination), gene flow, etc.

At the population level, genotypic variations are very important, because thanks to these, individuals have greater possibilities of adapting to new environmental conditions. However, not all genotypic variations produce successful phenotypes, many can be detrimental.

What are the genotypic variations?

Genotypic variations, as we have already mentioned, are those differences found in DNA sequences between individuals of the same population.

These variations can occur in the germ line (in the sex cells of sexually reproducing individuals) or in somatic cells, but only those that occur in the germ line are transmitted or fixed in the offspring.

When too many and very marked genotypic variations occur in a population of a given species, a new species may arise, which could eventually be reproductively isolated from individuals of other populations of the initial species.

Genotypic variations can be imperceptible or significant, but this depends on the magnitude of the variation and the genes that are affected by it.

These changes in the genotype of a population can be the product of different factors, among them mutations and recombination.

Types genotypic variations

The main genotypic variations are those that arise as a product of mutations or genetic recombination.

Mutations are permanent alterations in a DNA sequence and can occur for different reasons: errors during DNA replication that are not correctly repaired, due to viral infections, due to the effects of ultraviolet light, among others.

Mutations can be beneficial or deleterious, but this almost always depends on the environmental context where the individual that presents it develops.

Cells in the human body, for example, can undergo different types of mutations that can be harmless or can lead to the development of major diseases. However, only those mutations that occur in the germ line and pass to the offspring are those that influence the genotypic variations of a population.

Recombination, on the other hand, has to do with the mixing of genetic material between the homologous chromosomes of sexually reproducing individuals.

During this process, which occurs during meiosis, that is, during the formation of sex cells, homologous chromosomes (the one that comes from the mother and the one that comes from the father) are randomly crossed over and mixed in the cells of the germ line. .

What types of genotypic variations exist?

Single Base Pair Substitutions

This type of variation is also known as a single nucleotide polymorphism and can occur as a result of any substitution in the DNA, either by transition or transversion.

The transitions have to do with exchanges between purines or pyrimidines, that is, an adenine for a guanine (or vice versa) and a cytosine for a thymine (or vice versa). Transversions consist of the exchange, in a DNA sequence, of a purine base for another pyrimidine or vice versa, which modifies the sequence.

insertions or deletions

The insertions or deletions consist of the elimination or addition of DNA fragments of variable lengths in the nucleotide sequence of a gene, together these variations are known as «indels».

An «indel» can cause the synthesis of a defective protein (by translation, folding, translocation, post-translational modification, etc.) or a change in the open reading frame of a gene, which can mean, for example, the production of a totally different protein.

structural variations

Structural variations usually have to do with variations in large DNA sequences, very often referring specifically to changes in the number of chromosomes or to structural rearrangements of the same.

There are numerous variations of this type, among which are insertions, deletions, inversions, duplications, and variations in the number of copies of a chromosome.

Any of these chromosomal variations usually results in deleterious phenotypes or in which important functions are compromised.

Examples of genotypic variations

In natural populations we can find countless examples of genotypic variations, many of which are imperceptible at the phenotype level, but others can be distinguished with a simple look at the physical traits of individuals.

The obesity

In human populations, people who inherit the FTO gene from their parents have a greater than 70% chance of developing obesity. One in six individuals carrying this genotypic variation has 15% more adipose tissue and an average body weight 3 kg higher than individuals with the «normal» form.

sex

Sex is determined by the presence of a specific combination of sex chromosomes. Females receive an X chromosome from both parents, while males receive an X chromosome from their mother and a Y chromosome from their father.

Therefore, women and men are genotypically different with regard to these chromosomes and the information they contain.

triple X syndrome

A genotypic variation related to the number of chromosomes is what gives rise to the triple X syndrome, presented in some women who are characterized by having three X chromosomes instead of two.

«Superwomen» with this syndrome generally have atrophied sexual organs and diminished fertility, but are intellectually normal.

Other examples of genotypic variations are found among human populations such as:

– The color of the hair

– The color of the eyes

– The height

– The metabolism

– Intellectual capacity

– Albinism

– Sickle cell anemia

– The blood type

References

Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000. Genetic variation. Available from: ncbi.nlm.nih.gov
Armstrong, D., Burke, M., Emery, L., McArthur, J., Nightingale, A., Perry, E., … Saunders, G. (2020). The European Bioinformatics Institute. Retrieved August 14, 2020, from ebi.ac.uk
Klug, WS, & Cummings, MR (2006). Concepts of genetics. Upper Saddle River, NJ: Pearson Education.
Pierce, BA (2012). Genetics: A conceptual approach. macmillan.
Encyclopaedia Britannica. (2019). Retrieved August 14, 2020, from britannica.com

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