The dominance, in genetics, it is a concept that refers to the property that one of the members of a pair of alleles (that code for a particular phenotype) has to suppress the expression of the other when they are in the heterozygous condition.
Alleles (genes) are segments of the genetic material that enclose the nucleus of all eukaryotic cells, are found on chromosomes and are transmitted from one generation to the next through reproduction..
For example, in a population of individuals of human beings, a trait such as eye color can be determined by the expression of different forms of the same gene, which are called "alleles".
Animals inherit one allele from each of their parents for each trait.
If these alleles are the same, that is, if both parents transmit the same type of allele to their offspring, their offspring are homozygous (homo = equal). If one parent transmits one type of allele and the other parent transmits a different one, their offspring are heterozygous (hetero = different).
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The dark-eyed character, for example, is "dominant" over the light-eyed color (which is recessive), so an individual who inherits an allele that codes for dark eyes from his father and an allele that codes for dark eyes from his mother. for light eyes it will have the dark-eyed phenotype.
This individual, heterozygous for said character, can reproduce with a woman heterozygous for the same character and have a child with light eyes, who, in that case, will be homozygous recessive.
It was Gregor Mendel, a naturalist and religious considered today the "father of genetics", who in 1865 formulated the concept of dominance for the first time..
When studying pea plants, Mendel observed that some traits of the "pure" (homozygous) parent plants he worked with were also expressed by the offspring that resulted from the crossing of two lines with different traits. Therefore, he deduced that there were some heritable characteristics that dominated over others.
The classic experiment on which Mendel based his deductions consisted of crossing two plants with different phenotypes, some with purple flowers and others with white flowers. In this "first crossing" all the resulting plants (of the first generation or F1) had purple flowers..
After crossing plants of this first generation with each other (all with purple flowers), Mendel realized that in the second generation (F2) there were plants with purple flowers (whose characteristic he called "dominant") and a lower proportion of flowering plants. white (whom he called "recessive").
Although the relationships between genotype and phenotype are much more complex than the dominance and recessivity described by Mendel, these concepts laid the foundation for the birth of genetics as a science and have been widely exploited ever since..
Although dominance is often ascribed to a gene or a character, this is not actually an intrinsic property of genes, but rather describes the pattern that is observed when a phenotype that is associated with a single member of a pair of alleles is expresses in the phenotype of the forms homozygous Y heterozygous.
With the above it is understood that said pattern is subject to changes, which depend on the composition of the allelic pair (it applies to diploid organisms, for which the same individual presents two alternative forms of the same gene, or two alleles) and the character or trait under consideration.
Let us remember that the phenotype is "the form that is shown", also defined as the set of "visible" characteristics of an individual that result from the expression of the genes that make up its genotype and from their interaction with the environment that surrounds it..
In addition to the fact that the phenomenon of dominance affects the phenotype that results from the genetic combination of an organism, it also affects the way in which genes are transmitted from an individual to their offspring..
That is, in a set of individuals (a population), those genes that are characterized by being "dominant" over other genes (in an allelic pair where both genes code for the same phenotypic trait) are always in greater quantity or frequency than the recessive genes.
This is because natural selection has favored individuals with dominant alleles for a particular trait more than individuals with recessive genes, a fact that is subject to variations, usually depending on environmental conditions..
Mendel was very lucky when he analyzed the results of his experimental crosses and determined that the purple color "dominated" over the white color, since the dominance relationship between the two alleles of the same gene in an individual is not always so "direct" or "simple".
The findings of the “post-Mendelian” era of genetics have shown that there is more than one type of dominance relationship between two alleles, which we describe as: complete dominance, incomplete or partial dominance, codominance, etc..
What Mendel observed with the color of the flowers on his pea plants is an example of complete dominance..
In this type of genotype / phenotype relationship, the heterozygous phenotype (combining a dominant and a recessive allele) is indistinguishable from that observed in the parental homozygous phenotype (with both dominant alleles).
In other words, the phenotype corresponds only to the characteristics determined by the dominant allele..
Sometimes, however, the phenotype that is observed as a result of the crossing of two individuals is a type of "intermediate phenotype" between the phenotype of the homozygous dominant and that of the homozygous recessive..
Therefore, from the mixture of two homozygous individuals (one recessive and the other dominant for a given trait) the resulting offspring exhibits a phenotype that is "intermediate" between both, which is related to the "incomplete" dominance of the dominant gene over the recessive.
An example of incomplete or partial dominance can be inheritance of hair type (curly and straight). Individuals who are heterozygous for curly hair (dominant) and for straight hair (recessive) have an intermediate characteristic, which we know as “wavy hair”.
The phenomenon of codominance is slightly different from that of incomplete dominance that we have just described, since in the codominance in the phenotype of the offspring resulting from the crossing of two homozygous individuals, the characteristics determined by the two parental alleles are observed..
Codominance, then, is when the phenotype of both parents is expressed in the offspring. A good example of this is the blood group system (ABO) in humans, which are determined by the expression 2 of 3 possible alleles, which code for proteins A, B or none (O).
Two parents homozygous for the alleles TO Y B, let's say a father AA and a mother BB (which both give rise to the blood phenotypes A and B, respectively) transmit to their common offspring an allele A and a allele B, respectively, with which their children result from phenotype AB (from the genotype AB).
Another good example of codominance is sickle cell disease. This disease is characterized by the deletion of an amino acid in the peptide chain of the oxygen-transporting protein in red blood cells: hemoglobin..
The change in the conformation of this protein also causes a change in the shape of the red blood cells, which acquire a characteristic "sickle" shape, which makes them less capable of transporting oxygen..
The disease is due to a mutation in the gene that codes for hemoglobin. Those individuals with the disease are heterozygous for the mutation, so they inherit a "normal" allele from one parent and a "mutant" allele from the other..
Since it is a case of codominance, these individuals present a population of normal cells and another of sickle cells, since the two alleles (the dominant normal and the recessive mutant) are expressed in the heterozygote.
Only recessive homozygotes present the disease, since in these all the red blood cells that are produced are sickle-shaped.
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