The dominant or recessive character of a gene refers to its ability to determine a specific internal (genotype) or external (phenotype) characteristic in an individual.
Genes are responsible for determining many of our external physical characteristics as well as many conditions or strengths of our health, and even many traits of our personality.
This knowledge is due to Mendel's Law of Segregation, according to which each organism has two genes for each trait..
When we talk about external or visible characteristics, we talk about phenotype, while the genetic code (internal or not visible) is known as genotype.
Humans and some sexually reproducing animals have two copies of each gene, called alleles that can be different from each other. One allele comes from the mother and one from the father.
Today it is known that such differences can cause variations in the protein produced, be it in frequency, quantity or location..
Proteins affect traits or phenotypes, so variations in protein activity or expression can affect those traits.
However, the theory considers that a dominant allele produces a dominant phenotype in individuals who have a copy of the allele, regardless of whether it comes from the father, the mother, or both. In the graphic representation of these combinations, the dominant is written in capital letters.
A recessive allele produces a recessive phenotype only if the individual receives two recessive (homozygous) copies, that is, one from each parent. In its graphic representation it is written with a lowercase letter.
An individual with a dominant and a recessive allele (heterozygous) for a certain gene will have the dominant phenotype. In that case, they are considered "carriers" of the recessive allele..
This means that the recessive gene does not show up in the phenotype if the dominant allele is present. To express itself, it needs the organism to have two copies of it, one from each parent..
That is to say that when an individual (human or animal) receives a copy from each parent, it is known as a homozygous combination and, generally, it ends up manifesting itself in the phenotype, while if it receives different copies (dominant and recessive), one from each parent , the combination is heterozygous.
A dominant gene is expressed in both cases: homozygous or heterozygous.
It is important to note that although these considerations are useful in assessing the likelihood that an individual inherits certain phenotypes, especially genetic disorders, they do not fully understand how a gene specifies a trait..
This is because at the time these discoveries were made, there was no information on DNA..
Therefore, there is no universal mechanism by which dominant and recessive alleles act, rather it depends on the particularities of the proteins they encode..
The classic example of the characters of a gene is the one that has to do with the color of the eyes. The allele that determines the color brown, for example, is dominant (M); while the one that determines the blue color is a recessive allele (a).
If both members of a couple have brown eyes and eventually have children, they will have brown eyes because they are both contributing a dominant gene..
If it turns out that one of the members of the couple has blue eyes and the other has brown eyes, it is most likely that the children have brown eyes; unless they inherit recessive alleles from other family members.
On the other hand, if it turns out that both parents have dominant alleles (different from each other), the offspring will show new characteristics as a result of a mixture between the characteristics of the parents..
This means that when both members have dominant genes, there is no way for one to “dominate” the other, resulting in something new, different..
In the case of mice Chaetodipus intermedius, skin color is controlled by a single gene that encodes the protein that makes pigment dark; the dark coat allele is dominant and the light coat allele is recessive.
Although keratin proteins normally bind to form fibers that strengthen hair, nails, and other tissues in the body, there are genetic disorders, with dominant patterns, that involve defects in keratin genes such as congenital pachyokinia..
Blood group AB is the result of the codominance of the dominant A and B alleles. That is, two dominant alleles are combined and something new results.
It is a disease caused by genes related to sex. In this case, it is a recessive gene, so it takes two copies for a female to have the disease, while only one copy of the hemophilia allele is needed for a man to have it..
This is because females have two X chromosomes (XX), while males have one X chromosome and one Y chromosome (XY). For this reason, hemophilia is more common in men..
This is an inherited disease that causes pain and damage to organs and muscles, because the irregular shape of the cells of the red blood cells (long and pointed) often causes blockage in the blood flow by being trapped in the capillaries.
This disease has a recessive pattern and only individuals with two copies of the sickle cell allele have the disease..
In addition to causing disease, the sickle cell allele makes the person carrying it resistant to malaria, a serious disease caused by mosquito bites..
And such resistance has a dominant inheritance pattern. Only one copy of the sickle cell allele is sufficient to protect against infection.
Everything that has been said so far has led to advances that are controversial for many, such as in vitro fertilization (IVF), which allows a couple to have difficulty conceiving, fertilize the woman's ovum with "ideal" sperm or in the best conditions.
For many, this procedure is ethically questionable because it leaves open the possibility for the person to choose the characteristics of the sperm and the ovum used to "design" an individual with particular characteristics..
Something similar occurs with genetic engineering or genetic modification (GM), which consists of putting a gene from one organism in the cells of another, producing what is known as a transgenic organism.
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