The recessivity, in genetics, it is a type of relationship that occurs between the two alleles of a gene (in heterozygous condition), in which one of them (the recessive) is not expressed in a “visible” way due to “masking” or the influence of the expression of his allelic partner, who is dominant.
An allele is defined as the alternative form of a gene, which is nothing more than a segment of DNA that provides the information necessary for the establishment of a phenotype. In animals, which are diploid organisms, there are two alternative forms of each gene, one on each homologous chromosome..
The phenotype, on the other hand, is any "visible characteristic" of an organism that arises as a result of the expression of its genes and its interaction with the environment where it lives..
If an individual inherits the same allele for a gene from his father and mother, this is said to be homozygous, but if an individual inherits one allele from his father and another different from his mother, this is said to be heterozygous, in which case, the phenotype may correspond only to one of the two alleles (the dominant one).
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The first to describe dominance and recessivity was Gregorio Mendel when analyzing the results of crosses between pure lines of pea plants with purple flowers and with white flowers..
In the first generation of offspring, all the plants turned out to be purple flowers and once he crossed these individuals with each other, he observed a small proportion of individuals with white flowers, in addition to those with purple flowers..
In this case, Mendel determined that the recessive allele corresponded to the one that determined the white color and that the dominant one corresponded to the purple color..
Recessive alleles are not distinguishable by simply observing the heterozygous phenotype, since in this only the characteristics “imposed” by the dominant allele are observed.
The phenotype that these alleles confer is rarely observed in nature, and is only distinguishable in homozygous double recessives..
Experimentally speaking, determining the dominance or recessivity of a trait or of the gene that codes for a given trait is very difficult, since the phenotype shown is, in reality, the product of the interaction between many different genes and the environment that surrounds them..
The effect of a dominant gene on a recessive gene can be explained with a simple artistic analogy:
Suppose we have two paints in different jars: one black and one red, each representing an allele for the same character, that is, the color of the paint..
If in a new bottle we mix two equal drops of black color (dominant homozygous), the color that we will obtain will be black (dominant phenotype).
Now, if we mix two drops, one black and one red, we will also obtain the previous phenotype: black; but in this case this will be the product of the mixture of black and red (heterozygous).
Only if we mix two drops of red paint in a container will we get red as a result (homozygous recessive).
Note: Since in a mixture of colors we cannot separate those that we have united, which is the case with alleles, this analogy is not useful to explain the crossing between two heterozygous organisms, where they would be obtained:
- Dominant homocytodroplets (with black phenotype)
- Heterozygous (with a black phenotype)
- Homozygous recessive (with red phenotype)
The term "recessive" is used both to describe the gene whose expression is masked by that of its dominant allelic partner, and to describe the individual who carries both members of an allelic pair in the recessive condition (homozygous).
As is true for the phenomenon of dominance, recessivity is not an intrinsic property of a gene, but rather the phenotypic pattern or the relationship between the two alleles of a gene, in which the phenotype observed in the heterozygous corresponds to the expression of only one of the two.
Until he was finally able to verify the existence of “recessive alleles”, Mendel made hundreds of different crosses, since only by obtaining pure lines is it possible to determine the relationship of dominance and / or recessivity between the alleles of the same gene.
In molecular terms, the appearance of these alleles is due to two possible events:
Recessive alleles are generally mutated forms of a gene. The frequency with which these appear in the population in the homozygous form is much lower than that of heterozygotes and, even, than that of dominant homozygotes.
In many cases, homozygous recessive conditions are lethal or extremely harmful or disadvantageous for those individuals who present them, so why don't these genes disappear from the population allelic set?
The answer is quite simple, some of these individuals manage to reproduce, "escaping" for a moment from the effects of natural selection, so they are able to transmit their genes to the next generation..
Many human genetic diseases are recessive, which means that they are observed only when individuals have a homozygous genotype for the same trait in its recessive condition, generally mutant, which is sometimes defective..
There are multiple examples of these conditions that, in general, are autosomal recessive, that is, they reside on any of the chromosome pairs 1 to 22 (not on the sex chromosomes), so men and women can suffer them equally ( are not linked to sex).
On the other hand, many of these diseases involve a single gene, that is, only the presence of two recessive alleles (homozygosity) results in the appearance of the pathological phenotype..
Dominant homozygous people, that is, those who have the "wild" or "normal" form of the gene whose mutation produces a disease do not have symptoms or are carriers.
Meanwhile, those who are heterozygous, who have the wild phenotype, are called carriers, although they do not present symptoms and, finally, only double recessives suffer from the disease and are more likely to have offspring with the same condition..
- Sickle cell anemia
- Cystic fibrosis
- Huntington's disease
- Tay-Sachs disease
- Hemophilia A
Other inherited recessive conditions, although not necessarily considered a disease, include albinism and dwarfism..
Albinism has to do with the partial or complete absence of melanin production, the pigment that determines the coloration of the skin, hair and eyes, and which also participates in the development of the ocular nerves..
This condition is caused by a mutation in one of the genes that works in the metabolic pathway for the synthesis of said pigment. The alleles that cause albinism are inherited in a Mendelian manner and are only seen in homozygous recessive individuals..
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