He 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 Segregation Law, according to which each organism has two genes for each trait.
When talking about external or visible characteristics, we speak of phenotype, while the genetic code (internal or non-visible) is known as genotype.
Nature of dominant and recessive character
Human beings and some animals, of sexual reproduction, have two copies of each gene, denominated alleles that can be different from each other. One allele comes from the mother and another from the father.
It is now known that such differences can cause variations in the protein it produces whether in frequency, quantity or location.
Proteins affect traits or phenotypes, so variations in the activity or expression of proteins 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 copies (homozygous), that is, one from each parent. In its graphic representation it is written with lowercase letter.
An individual with a dominant allele and a recessive (heterozygous) allele for a given 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 manifest itself in the phenotype if the dominant allele is present. In order to express itself, it needs the organism to possess two copies of it, coming from one of each parent.
That is to say, when an individual (human or animal) receives a copy of each parent, it is known as homozygous combination and usually ends up manifesting in the phenotype, whereas if it receives different copies (dominant and recessive), one of 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 for assessing the likelihood that an individual inherits certain phenotypes, especially genetic disorders, this does not allow a complete understanding of how a gene specifies a trait.
This is due to the fact that at the time these discoveries were made, there was no information on the DNA .
Therefore, there is no universal mechanism by which dominant and recessive alleles act, but depends on the particularities of the proteins they encode.
Examples of a dominant or recessive gene
Color of eyes
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 brown color, for example, is dominant (M); While the one that determines the blue color is a recessive allele (a).
If both partners have brown eyes and have children, they will have brown eyes because they are both contributing a dominant gene.
If it turns out that one of the partners has blue eyes and the other has brown, the children are more likely to have brown eyes; Unless they inherit recessive alleles from other members of the family.
If, on the other hand, both parents have dominant alleles (different from each other), offspring will show new characteristics as a result of a mixture of parent characteristics.
This means that when both members have dominant genes, there is no way for one to"dominate"the other so that something new, different.
Skin color
In the case of mice Chaetodipus intermedius , The color of the skin is controlled by a single gene encoding the protein that makes the pigment dark; The dark coat allele is dominant and the light coat allele is recessive.
Keratin
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 paquiochia.
Blood type
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.
Hemophilia
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 a copy of the allele of hemophilia is needed for a man to suffer.
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.
Sickle Cells
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 the disease, the sickle cell allele makes the bearer, is 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.
Some implications
All that has been said so far has resulted in advances that are controversial for many, such as IVF, which allows a couple to have difficulty conceiving, fertilizing the woman's egg with"ideal"sperm or The best conditions.
For many, this procedure is ethically questionable because it leaves open the possibility that the person chooses the characteristics of the spermatozoa and the egg used to"design"an individual with particular characteristics.
Something similar happens with genetic engineering or genetic modification (GM), which involves putting one gene from one organism into the cells of another, producing what is known as a transgenic organism.
References
- BBC (2011). Dominant and recessive alleles. Retrieved from: bbc.co.uk.
- Mendelian inheritance (s / f). Dominant and recessive genes. Retrieved from: herenciamendeliana.blogspot.com.
- Learn genetics (s / f). What are Dominant and Recessive?. Retrieved from: learn.genetics.utah.edu.
- Pérez, Guillermo (s / f). Dominant and recessive phenotype. Recovered from: fenotipo.com.
- Vaiva (2015). Difference between dominant and recessive. Retrieved from:
- Your Genome (2016). What are dominant and recessive alleles. Retrieved from: yourgenome.org.