Heterozygous: what it is, characteristics and how it affects reproduction

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

This can be achieved by having children (classic fitness) or helping very close relatives to have them (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, infinite reproductive strategies emerge in the natural world that try to maximize the cost/benefit of having children: partitioning, binary fission, autotomy, polyembryony, hybridization and many other forms further. Environmental impositions modulate the behavior of living beings and, therefore, their reproductive strategy.

In any case, to understand genetic segregation in a population from the beginning, you have to go back to basics. Keep reading, here We tell you everything about what it means to be heterozygous, at the individual and population level.

• Related article: "The 4 types of sex cells"

The bases of genetics in living beings

Before incurring in the zygosity of living beings, it is necessary to clarify a series of terms that can cause confusion. First of all, it should be noted that Human beings are diploid (2n), that is, we have a copy of each chromosome within 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 the product of sexuality, since in our species (and in almost all of them), it comes from the union of a haploid sperm (n) and a haploid ovum (n). Each of these gametes are diploid in their initial maturation phase (germ stem cell), but thanks to meiosis, it is possible to reduce the genetic load in half. Thus, when two gametes are fertilized, the zygote recovers diploidy (n+n=2n). As you can imagine, each of the two chromosomal copies of the zygote belong to one of the parents.

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

• An allele is dominant (A) when it is expressed in the individual's phenotype 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 of the homologous chromosome is the same as it.
• Homozygous dominant (AA): the alleles in the chromosomal pair are exactly the same and, moreover, dominant. The dominant trait (A) is expressed.
• Homozygous recessive (aa): the alleles in the chromosomal pair are exactly the same and, moreover, 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 (A) phenotype is expressed.

What does it mean to be heterozygous?

With this little class of genetics, 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.

Actually, Many characters are oligogenic or polygenic, that is, they are influenced by more than one gene and the effects of the environment.. In these cases, the phenotype variation is not explained by zygosity alone.

In addition, it is also possible that both alleles are expressed at the same time, to a lesser or greater proportion, through a biological phenomenon known as “codominance”. In this specific picture, there is not a dominant allele (A) and a recessive one (a), but both are part of the phenotype or external characteristics of the carrier.

The heterozygous advantage

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

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

Let's give 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 presents a healthy CFTR gene (A), it will be able to overcome the lack of its other mutated copy and allow the person to be fury. Therefore, a person who is 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 so sometimes the lack of functionality of one of the two alleles can generate some quantifiable mismatches. In any case, for an autosomal recessively inherited disease to manifest itself in all its splendor, it is necessary that both copies of the affected gene are mutated and, therefore, be dysfunctional (aa). That 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.

Based on this mechanism, it is not difficult for us to understand why there are breeds of dogs and cats with serious health problems (up to 6 out of 10 Golden Retrievers die from cancer and up to half of Persian cats have polycystic kidneys, for example). The more parents are reproduced among them, the more the tendency is towards homozygosity, and the more it is promoted that two deleterious recessive alleles end up joining in the genotype of the same individual. This is why inbreeding is associated with long-term disease and death in a population..

In addition, it is possible that heterozygosity gives the offspring more evolutionary advantages than those carried by its offspring. parents being homozygous, or what is the same, the phenotype of the heterozygote is more than the sum of their parts. For this reason, 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 be homozygous. The less variability there is between individuals, the more likely it is that everyone will be affected in the same way. before a deleterious change.

Summary

Despite everything described, we want to emphasize that at all times we have 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 endured over time? The paradox of asexuality in "evolutionarily simple" beings is not yet resolved, but it is clear that the homozygosity derived from the reproduction between consanguineous is negative for practically all the vertebrates.

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