The incomplete dominance, also known as semi-dominance or intermediate inheritance, is when the alleles of a gene that determines a certain characteristic in an individual do not have a relationship of dominance and recessivity, but rather the characteristic that we observe is a combination of the characteristics determined by both alleles.
To better understand this concept, it is first necessary to know a little more about genes and alleles..
The genes are the segments of a molecule that we know as deoxyribonucleic acid (DNA), which is found inside the nucleus of each of our cells, and those of other animals and plants, in the form of chromosomes.
All the characteristics of our body and of the cells that compose it are determined by the information contained in our DNA and the same happens with the rest of the living beings on Earth. We inherit the DNA of our parents.
The color of our eyes, the shape of our hair, the color of our skin, the shape of our nose and ears, our metabolism and our immune system, etc., are determined by our genes..
We call these visible features phenotype and we say that our phenotype is the result of the expression of our genotype, that is, of the genes that determine each characteristic.
For example, the "wavy hair" phenotype is determined by a combination of alleles that determine the "hair type".
The genes that we have in our cells are inherited from our parents through sexual reproduction, where from each one we obtain half of all genes.
Alleles are the different forms that a gene can have. In a population of organisms the same trait occurs in different ways, since not all individuals have the same combination of alleles for each trait.
Human beings, animals and other living beings that reproduce sexually receive an allele from each of our parents.
When both alleles are of the same type - they are the same - we say that we are homozygous for the trait they determine; on the other hand, when the two alleles are different we say that we are heterozygous.
When the alleles for a trait are the same, the phenotype that we observe corresponds exactly to that encoded by these genes, but when they are different, what we are going to observe will depend on the relationship of dominance and recessivity between both alleles..
The dominance and the recessivity refer to the relationship that exists between the expression of the alleles of each gene that we have in our cells.
We can only know what is the phenotype that encodes this allele when it is in the condition of homozygousness, that is, when the individual is homozygous for this allele (he has two recessive alleles of the same type).
Although it is not a norm, recessive alleles tend to be less common in populations or, in other words, they tend to be less frequent than dominant alleles.
The notions of dominance Y recessivity arose in 1856 with the works of Gregorio Mendel, today considered the "father of genetics".
Mendel worked with different features or characters of the pea plants (Pisum sativum) and he devoted himself to the study of the transmission of these traits between generations, where he realized that there were some traits that “dominated” over others when homozygous plants for certain traits were crossed..
The easiest way to understand dominance and its variations is with a Punnett square, which is nothing more than a graphical method to facilitate genetic analysis..
In this type of diagram, alleles are generally represented by letters: dominant with uppercase and recessive with lowercase. Let's look at a typical example of dominance:
| Crossing | C (yellow seed) | C (yellow seed) |
| c (green seed) | Cc (yellow seed) | Cc (yellow seed) |
| c (green seed) | Cc (yellow seed) | Cc (yellow seed) |
In his works, Mendel noticed that the yellow color of the seeds dominated about the green color. This is: when I crossed plants that produced only yellow seeds with plants that only produced green seeds, 100% of the resulting plants produced yellow seeds..
Thus, in the Punnett square above, the allele for the color yellow (C) is dominant over the allele for the color green (c), so that all the descendants of a cross of CC plants (with yellow seeds) and plants cc (with green seeds) are heterozygous Cc (with yellow seeds).
Although Mendel did not observe something different, today we know that different types of dominance and recessivity relationships can occur between the alleles of a gene, as is the case of the incomplete dominance.
Mirabilis jalapa, better known as "Dondiego at night" (or in English as four-o'clocks), is a species of ornamental shrubby plant cultivated in Central and South America that produces beautiful flowers of different colors.
This type of plant is noted for the incomplete dominance relationship that can be observed with respect to the alleles that encode the color of its flowers.
When homozygous plants for white flowers (BB) and homozygous plants for red flowers (RR) are crossed, the phenotype of the resulting plants is a mix of both colors, that is, the plants have pink flowers (BR).
As the resulting color of the flowers in heterozygous individuals is pink, we say that this phenotype is intermediate with respect to the other two phenotypes, so we cannot say with precision which of the two (white or pink) is the one that dominates over which.
Many plants in nature have this type of relationship between their alleles. Color is determined by pigments and these pigments are produced by proteins that are encoded by genes..
In incomplete dominance, both alleles produce a certain amount of proteins that, in turn, produce a certain amount of pigment, hence the phenotype that we observe is a mixture.
Another case of incomplete dominance can be that of the hair shape in humans..
If a man homozygous for curly hair has a child with a woman homozygous for straight hair, it is most likely that one of their children has wavy hair, which is neither completely curly nor completely straight, but a combination of both characteristics..
Some chickens with blue plumage result from crossing chickens with black plumage and chickens with white plumage..
In this case, as in the case of the color of the flowers and the shape of the hair in humans, neither allele dominates the other, but the phenotype observed is a combination of the characteristics determined by each allele.
There are aubergines that bear white fruits and aubergines that bear deep purple fruits. Incomplete dominance is observed when two plants of these types are crossed, resulting in fruits of an intermediate color, that is, light purple.
This disease is due to a genetic disorder that causes abnormally high levels of low-density lipoprotein cholesterol in the blood, leading to early cardiovascular disease.
In this disease, one of the alleles causes the liver cells to be devoid of cholesterol receptors, while the other produces normal liver cells with respect to these receptors.
The phenotype observed in heterozygotes corresponds to cells with low numbers of receptors, that is, an intermediate phenotype that is not capable of removing all cholesterol from the blood..
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