Heterozygote: what is it, characteristics and how does it affect reproduction?

Let's see what a heterozygote consists of and how it relates to the functioning of genes.

Genetics is the answer and the engine to life itself. As indicated by multiple theories and postulations (from the origin of species to Kin Selection), animals do not live for the enjoyment of existence or for a higher purpose, but their only vital goal is to leave offspring, or in other words, to increase the proportion of their own genes in the next generation.

This can be achieved by having children (classical fitness) or by helping close relatives to have children (inclusive fitness).

The basis of all these concepts lies in reproduction, whether sexual or asexual.If a living being cannot leave offspring in any way, it is impossible for its genetic imprint to be expressed in subsequent generations. Based on this premise, an infinite number of reproductive strategies arise in the natural world that try to maximize the cost/benefit of having children: partitioning, binary fission, autotomy, polyembryony, hybridization and many other forms. Environmental impositions modulate the behavior of living beings and, therefore, their reproductive strategy.

However, to understand genetic segregation in a population from the beginning, we have to go back to basics. Read on, because here we tell you all about what it means to be heterozygous, at the individual and population level..

The basics of genetics in living things

Before going into the zygosity of living beings, it is necessary to clarify a series of terms that can generate confusion. In the first place, it is necessary to emphasize that humans are diploid (2n), that is, we have one copy of each chromosome in the nucleus of each and every one of our somatic cells.. Thus, if we carry 23 chromosome pairs at the cellular level, the total karyotype will be 46 (23x2=46).

Diploidy is a product of sexuality, since in our species (and in almost all), this comes from the union of a haploid sperm (n) and a haploid egg (n). Each of these gametes is diploid in its initial maturation phase (germinal stem cell), but thanks to meiosis, it is possible to reduce the genetic load by half. Thus, when two gametes are fertilized, the zygote regains diploidy (n+n=2n). As you can imagine, each of the two chromosomal copies of the zygote belongs to one of the parents.

Now, another extremely important concept comes into play: the allele. An allele is each of the alternative forms that can have the same gene, whose difference lies in the sequence of nucleotides that compose it.. A human being will have two alleles for each gene, since one will be located in a section of the paternal chromosome and the other in the homologue of the maternal chromosome. From here, we can highlight a number of generalities:

• An allele is dominant (A) when it is expressed in the phenotype of the individual regardless of the nature of the copy on the homologous chromosome.
• An allele is recessive (a) when it is expressed in the phenotype of the individual if and only if the copy on the homologous chromosome is equal to it.
• Homozygous dominant (AA): the alleles in the chromosomal pair are exactly the same and, in addition, dominant. The dominant trait (A) is expressed.
• Homozygous recessive (aa): the alleles in the chromosomal pair are exactly the same and, in addition, recessive. It is the only case in which the recessive trait (a) is expressed.
• Heterozygous (Aa): the alleles are different. Because the dominant (A) masks the recessive (a), the dominant phenotype (A) is expressed.

What does it mean to be heterozygous?

With this little genetics lesson, we have defined heterozygosity almost without realizing it. A diploid (2n) individual is heterozygous when a given gene within the nucleus is composed of two different alleles.In typical Mendelian genetics, one dominant (A) and one recessive (a). In this case, the dominant allele is expected to be expressed over the other, but the genetic mechanisms are not always so simple to explain.

In fact, many traits are oligogenic, many traits are oligogenic or polygenic, i.e., they are influenced by more than one gene and the effects of the environment.. In these cases, phenotype variation is not explained by zygosity alone.

In addition, it is also possible for both alleles to be expressed at the same time, in lesser or greater proportion, through a Biological phenomenon known as "codominance". In this particular case, there is not a dominant allele (A) and a recessive allele (a), but both are part of the phenotype or external characteristics of the carrier.

The heterozygote advantage

In population genetics, diversity is often synonymous with the future.. A population nucleus of a given species where all the individuals are almost the same at the genetic level has a very poor prognosis, because at the slightest environmental change, it is possible that this genome will cease to be completely useful in the immediate future. If all specimens are the same, they will react in the same way, for better or worse.

For this reason, it is thought that heterozygous individuals (and populations with a high rate of heterozygosity) have an advantage over those homozygous for the same gene. Better said, the more genes with two different alleles a specimen has, the more likely it is that its genetic plasticity will be more suitable.. This is not merely a conjectural concept, since it has been demonstrated that homozygosity is a product of inbreeding (reproduction between consanguineous), something clearly harmful in the natural environment.

Let us take an example. Cystic fibrosis is a clinical entity caused by the CFTR gene, located on the long arm of chromosome 7, at position 7q31.2. This mutation is recessive (a), because if the homologous chromosome has a healthy CFTR gene (A), it will be able to compensate for the lack of its other mutated copy and allow the person to be healthy. Therefore, a person heterozygous for the disease gene (Aa) will be a carrier, but will not manifest the clinical picture, at least in most autosomal recessive diseases.

This is not always the case, as sometimes the lack of functionality of one of the two alleles can generate some quantifiable mismatches.. In any case, for an autosomal recessive disease to manifest itself in all its splendor, it is necessary for both copies of the affected gene to be mutated and therefore dysfunctional (aa). This is why heterozygosity is less associated with disease than homozygosity, since a recessive pathology can be masked, even when the patient carries the affected gene.

According to this mechanism, it is not difficult to understand why there are breeds of dogs and cats with serious health problems (up to 6 out of 10 Golden Retrievers die from cancers and up to half of Persian cats have polycystic kidneys, for example). The more parents reproduce with each other, the more homozygosity tends to occur, and the more it is promoted that two deleterious recessive alleles end up joining together in the genotype of the same individual. This is why inbreeding is associated with long-term disease and death in a population..

Furthermore, it is possible that heterozygosity gives the child more evolutionary advantages than those carried by the homozygous parents, i.e. the phenotype of the heterozygote is more than the sum of its parts. Therefore, at the genetic level, it is often said that a population with a high percentage of heterozygosity in the genome is "healthier" than one with a tendency to homozygosity. The less variability there is between individuals, the more likely it is that they will all be affected in the same way by a deleterious change. to a deleterious change.

Summary

In spite of all that has been described, we would like to emphasize that we have always moved on general grounds, since for each rule there is at least one meaning in the natural environment. We operate on the basis that genetic diversity is the key to success, but then, why are there genetically identical asexual species that have remained in time? The paradox of asexuality in "evolutionarily simple" beings is not yet resolved, but it is clear that homozygosity derived from inbreeding is negative for virtually all vertebrates.

Thus, we can affirm that, in a natural population, genetic variability is the key to success. For this reason, in population genetic studies, it is generally it is often generalized that a high percentage of heterozygotes is synonymous with health..