Genetic segregation: what it is, characteristics and examples

Genetic segregation is a concept that can help us understand how genes are expressed.

Living organisms adopt two types of basic life strategies for the conception of offspring: asexual and sexual reproduction.

In asexual reproduction, a cell or a group of cells of a parental organism gives rise to another functional individual, genetically equal to its parent. This is achieved through bipartition, budding, polyembryony, parthenogenesis and other complex processes.

On the other hand, in sexual reproduction there are individuals of two genders within a species: males and females. Both produce gametes with half the genetic information of the rest of the cells (they are haploid) and, when they come together, give rise to a zygote that regains its normal chromosomal number (diploidy). This process is much more costly than the previous one, but it has a number of advantages that alone explain evolution.

In asexual reproduction, all offspring are the same as the parental organism. In sexual reproduction, on the other hand, each offspring has a different genetic makeup, since half of its chromosomes are maternal and the other half paternal. Due to overcrossing, chromosomal permutations and other processes that take place during meiosis, no offspring is the same as its sibling (unless they are twins). Here's what it's all about genetic segregation has to do with all these terms.

What is genetic segregation?

If you have been interested in genetics at some point in your life, you are probably familiar with Gregor Mendel. This Augustinian friar, Catholic and naturalist, formulated thanks to his experiments with peas (Pisum sativum) the well-known Mendel's laws, published between 1865 and 1866. Unfortunately, these documents did not begin to gain notoriety in scientific culture until 1900, when Mendel had already passed away.

For his part, the term "genetic segregation" refers to the distribution of genes from parents to offspring during meiosis, i.e., the reason for the distribution of genes from parents to offspring during meiosis.i.e. why the genome of the offspring results from the union of different parents. To exemplify the mechanisms of gene segregation, it will be of great help to briefly review Mendel's three laws, which is why we have made a special mention to his figure.

Since we are going to immerse ourselves in Mendel's world, we must lay certain foundations. First of all, it should be noted that we are going to focus on diploid beings, i.e., animals and plants that have two sets of homologous chromosomes of each type (2n) in their nucleus. If the human being has 46 chromosomes inside each cell, 23 come from the mother and 23 from the father.

Within each chromosome, there is a series of ordered sequences of DNA that have the information needed to synthesize proteins or RNA: genes.. On the other hand, each gene can present different "forms" that depend on the sequence of nucleotides, which are called alleles. Since we have two chromosomes of each type in our cell nuclei, we say that we also have two alleles for each gene.

A particular allele, according to typical Mendelian genetics, can be dominant (A) or recessive (a). Dominant alleles are those that are expressed independently of their partner (AA or Aa), while recessive alleles require both alleles to be the same for the same gene (aa). For a given gene, an individual can be homozygous dominant (AA), homozygous recessive (aa) or heterozygous (Aa). In the latter case, the dominant trait (A) is expressed and the other is masked (a).

With these ideas in mind, it only remains for us to clarify that the genotype is the set of genetic information in the form of DNA carried by a particular living being, while the phenotype is the part of that gene in the form of DNA.while the phenotype is the part of that genome that is expressed at the visible level.

At this point, it should be emphasized that the phenotype is a product of the environment and the genes, so the genome is not always the product of the environment.so that the genome does not always fully explain the external traits. Now, let us look at Mendel's laws.

Phenotype: genotype + environment

1. Principle of uniformity (first generation)

Let's take a fictitious example that departs a little from the typical Mendelian pea seeds. Imagine with us, for a moment, that a species of bird has in its genome the COL1 gene, which codes for feather coloration.

In turn, this gene has two variants: COL1A and COL1a. The first allele (A) is dominant and manifests itself at the phenotype level with a red hue, while the second allele (a) is recessive and manifests itself with a yellow color.

According to the principle of uniformity, if two homozygous parents come together (one has both AA alleles and the other has both aa alleles), all the offspring will be heterozygous (Aa) for that gene, without exception. for that gene, without exception. Thus, one of the parents will be red (AA), the other will be yellow (aa) and all the offspring will also be red (Aa), since the red trait is superimposed on the yellow one.

2. Principle of segregation (second generation)

Let us now see what happens if this red generation (Aa) is reproduced among them. First we apply the formula and then explain the result:

Aa x Aa= ¼ AA, ¼ Aa, ¼ Aa, ¼ aa

According to these values, if two heterozygotes for a given gene are crossed, 1 out of 4 offspring will be homozygous dominant, 2 out of 4 will be heterozygous and 1 out of 4 will be homozygous recessive..

If we go back to our example, we will see that from two mated red parents, three out of four offspring are also red (the Aa and AA), but one of them recovers the yellow phenotype (aa), which was masked in the previous generation.

Thus, the frequency of the red trait is distributed in the population at a ratio of 3:1. With this very basic statistical inference, it is demonstrated that the parental alleles are segregated during gamete production by meiotic cell division..

3. Principle of independent transmission (third generation).

To see how the alleles are distributed if we cross members of the third generation with each other, we would need a table with a total of 16 spaces, since each variant (AA, Aa, Aa and aa) can reproduce with any of the others (4x4:16).

We are not going to focus on these results, since it is clear from the previous example that the dominant red trait is the one that will prevail in the color of the feathers of our birds.

In any case, we are interested in rescuing an idea of the principle of independent transmission: the different traits encoded by different genes are inherited independently.that is to say, the inheritance pattern of the trait "feather color" that we have shown you does not necessarily affect the trait "beak size". This only applies to genes that are on different chromosomes, or at considerable distances within the same chromosome.

Limitations of genetic segregation postulations

Although these laws laid the foundations of what we know today as genetic inheritance (and therefore molecular genetics and all aspects of the discipline), it must be recognized that they fall a little short of what is known today.

For example, these postulations do not take into account the effect of the environment on the phenotype (external appearance of the specimen) and the genotype (its genome).. If the action of the sun's rays causes the feathers of our birds to fade (something without any basis, just to exemplify), it is possible that the phenotype of red birds becomes orange, not red. Despite being specimens with AA or Aa alleles for the COL1 gene, the environment modifies the external and visible.

It is also possible that feather color is encoded by the interaction between several genes, such as COL1, COL2, COL3 and COL4. Imagine, in addition, that one of them has a greater predominance over the rest and is more determinant for the final phenotype. Here 8 different alleles and very complex genetic issues come into play that cannot be explained only with Mendel's laws, so it would be necessary to enter into quantitative genetics.

As a final clarification, we would like to make it clear that all the examples cited here are fictitious, since we have no knowledge of whether there really is a COL1 gene that codes for one shade or another in a species of bird in nature. The human being has about 25,000 genes in its genome, so imagine having to say that there is a COL1 gene coding for one shade or another in a species of bird in nature.So imagine having to affirm or deny the existence of phenotypes and genotypes in many other wild species that have not even been sequenced.

What we want to make clear is that, with these laws of genetic segregation that we have shown you through examples, the separation of alleles during gamete production by meiotic cell division in reproductive events is explained. Although many traits are not governed by these mechanisms, they are always a good starting point to begin the study of genes, whether at the informative or professional level.