Genetics explains all physiological phenomena of living beings, from the origin of life to evolutionary radiation in specific parts of history, through diseases, genetic conditions, facial features and much more. Each of our cells has DNA inside and, without it, cellular metabolism and existence itself would be impossible.
Although it may seem difficult to understand, the transformation of the genome’s instructions into proteins encodes our external traits. DNA, packaged in the form of chromosomes within the cell, contains a series of nucleotides of great interest, specifically in genes. These DNA sequences are “copied” (transcribed) into messenger RNA and this travels to the cytoplasm.
The messenger RNA is “read” (translated) by ribosomes, which assemble amino acids according to the order established by the genes, to give rise to the necessary proteins. Thus, the genome is transformed into living tissues.
With this express protein synthesis class, we are prepared to show you the differences between genotype and phenotype which separates the genetic code from individual characteristics.
All human cells are derived from the division of a zygote, a cell resulting from the union of a female gamete (egg) and a male gamete (sperm). Each gamete is haploid, which means it has half the genetic information expected (n=23) to, when joined with its partner, give rise to a functional diploid organism (2n=46).
Our species is diploid and has 46 total chromosomes in the nucleus of each somatic cell, 23 from the mother and 23 from the father. Diploidy is a clear evolutionary adaptation, since by presenting “double” genetic information, we also have two copies of each gene. Each of the forms that a gene can acquire is known as an “allele”, differentiated from the rest by its sequence. So that, We inherit two alleles for each gene, one from the father and one from the mother
What are the differences between genotype and phenotype?
This introduction was necessary, because from here we are going to move into much more complex areas of alleles, genes, characters and terms. Next, we show you the differences between genotype and phenotype.
1. The genotype is not seen externally, but the phenotype is
The genotype is the set of genetic information that a particular organism has in the form of DNA This includes genes, extragenic regions, and gene-related sequences that do not encode proteins as such. In other words, the genotype is the set of nucleotides arranged in the form of the double helix of DNA that differentiates us from other species and from other individuals of the same species.
On the other hand, the phenotype refers to the set of physical and behavioral traits that an individual exhibits based on their genotype (it is not an ethereal concept, but a visible one). In any case, in the subsequent sections we will see that the phenotype is not only the manifestation of the genotype, despite what is argued in general sources.
2. The phenotype is not just a product of the genotype
Mendel’s laws and Mendelian genetics have been very useful in paving the way for modern geneticists, but in many areas they fall short Let’s take an example: a pea may have an allele (a) that codes for a rough skin, while the allele (A) codes for a smooth skin. The pea (aa) will be rough and the pea (AA) smooth, right?
So, what happens if we put the pea (AA) under water for 3 hours a day and its surface ends up becoming rough due to water stress? With this case, we want to exemplify that genotype alone does not explain phenotype, since the environment, nutrition and epigenetic mechanisms promote the appearance of variability beyond the alleles. A pea (AA) can have wrinkled skin due to environmental stressors, not because its genome dictates it.
The same premise is maintained when analyzing genetically identical people, such as twins (apart from sporadic mutations). Although many hereditary disorders occur in both at the same time (such as personality disorders), other times one of the twins may have a psychiatric pathology and the other does not. The same goes for weight, skin tone, muscles and many other things. If one leads a different lifestyle from the other and lives in a different region, they will naturally show a different genotype than their twin brother.
3. A character is not always coded by a single gene
Generally, An organism is said to be homozygous for a trait when both alleles (those on the chromosome of each parent) are the same In other words, if there is the possibility of inheriting a dominant allele (B) and a recessive allele (b), an individual can be homozygous dominant (BB), homozygous recessive (bb) or heterozygous (Bb). In the latter case, the allele that is expressed will be the dominant one (B).
In the most deterministic explanations of genetics, each pair of alleles (bb, BB, or Bb) will encode the nature of a specific trait, such as eye color. Even so, it is shocking to discover that an apparently simple character like eye color is influenced by more than 3 genes: EYCL1, EYCL2, EYCL3 (more than 6 alleles).
Anyway, some diseases are monogenic, that is, they are explained by the dysfunctionality of one or both alleles within the same gene. Sickle cell anemia (mutations on chromosome 11), cystic fibrosis (mutations on chromosome 7), and hemochromatosis (mutations on chromosome 6) are some examples of monogenic diseases.
4. Genotype and phenotype do not always agree
If we continue with the train of thought from the previous example, we will discover that many inherited diseases are autosomal recessive, that is, if there is a non-mutated allele (D), the one that is dysfunctional (d) will be masked. When a condition is recessive in nature, only the descendants will express it (dd) while those (Dd) will be carriers or will show a much more attenuated signology.
For this reason we have said before that diploidy is an adaptive condition. When the gene on the chromosome of one of the parents fails (the mutated allele), it is expected that the copy on the chromosome of the other parent will be able to overcome the deficiency. In any case, there are some diseases that are dominant, and in them it is enough for one of the genes to be mutated for the condition to manifest in its entirety.
5. Sometimes the phenotype is more than the sum of its parts
We have told you that a living being can be homozygous or heterozygous for a character. Heterozygotes always show the character encoded by the dominant allele (A), while the recessive one (a) is masked However, as with everything in life, there are exceptions that prove the rule.
Sometimes alleles are co-dominant and expressed at the same time, forming a character that is different from the sum of its parts. For example, if the gene for the color of the petals of a flower presents variability in the form of a white allele (w) and a red allele (W), if both are co-dominant, the specimen (Ww) will have petals with white and red spots. .
Summary
As a small note, it should be noted that all the alleles that we have named have received an arbitrary name for purely informative purposes, but they do not correspond to an established and standardized genetic designation in all cases. What we want to show when using these symbols is that a recessive allele is represented with a lowercase letter and a dominant one with a capital letter, nothing more.
Beyond technicalities, it is necessary to emphasize that the phenotype is not always the exact manifestation of the genotype. The environment, nutrition, exercise and epigenetic mechanisms (activation or inhibition of genes throughout life) also play an essential role in the variability of traits in the human population.
