WHAT IS A PURE LINE? (BIOLOGY) - BIOLOGY - 2025

A pure line in biology is a lineage that does not segregate, that is, those individuals or groups of individuals that when reproducing give rise to others identical to those of their class. This does not necessarily mean individuals of a clonal lineage, although they are essentially the only ones who could be "pure".

There are plants, for example, that can be reproduced vegetatively by cuttings. If several cuttings are planted from the same plant, theoretically we are creating a small pure population.

Punnet box. IVAN S. ESCOBAR, via Wikimedia Commons

If we take one of them and reproduce it when it reaches the adult stage in the same way and for several generations, we will have created a clonal lineage.

However, strangely enough, humans have always been more attracted to the generation of pure lines of organisms that reproduce sexually.

In these cases, a pure line is one in which no segregation is observed for a particular character or group of characters. That is, these "preferred" characters will always manifest in the same way, unchanged for generations.

Pure line in biology: homozygotes

For a geneticist, a pure line is one made up of homozygous individuals. Therefore, in diploid individuals, at the particular locus of the gene of interest, each homologous chromosome will carry the same allele.

If the line is pure for more than one genetic marker, this criterion will be the same for each of the individual genes for which the individual will be homozygous.

Recessive homozygous

When a preferred trait manifests itself from the manifestation of a recessive allele in a homozygous condition, we can have greater certainty of the purity of the line.

By observing the individual manifesting that associated character, we can immediately infer his genotype: aa, for example. We also know that to preserve this same character in the progeny we must cross this individual with another aa individual.

Dominant homozygous

When the pure line involves dominant genes, the matter is a bit more complicated. Aa heterozygous and AA dominant homozygous individuals will manifest the same phenotype.

But only homozygotes are pure, since heterozygotes will segregate. In a cross between two heterozygotes (Aa) showing the trait of interest, a quarter of the offspring could manifest the unwanted trait (genotype aa).

The best way to demonstrate the purity (homozygosity) of an individual for a trait that involves dominant alleles is by testing it.

If the individual is AA homozygous, the result of crossing with an aa individual will give rise to individuals phenotypically identical to the parent (but with Aa genotype).

However, if the individual tested is heterozygous, the offspring will be 50% similar to the tested parent (Aa) and 50% to the recessive parent (aa).

Pure lines in genetic improvement

We call genetic improvement to the application of genetic selection schemes aimed at obtaining and prevalence of particular genotypes of plants and animals.

Although it can also be applied to the genetic modification of fungi and bacteria, for example, the concept is closer to what we do to plants and animals for historical reasons.

Domestication of the living

In the process of domestication of other living beings, we devoted ourselves almost exclusively to plants and animals that served us as sustenance or companionship.

In this domestication process, which can be seen as a continuous process of genetic selection, we create a set of genotypes of plants and animals which, later, we proceed to "improve".

In this improvement process we have proceeded to obtain pure lines in terms of what the producer or consumer needs.

Plants

The plants thus improved are called varieties (in this case, commercial varieties) if they have been subjected to a scheme of tests that demonstrate their purity.

Otherwise, they are called types- and are more associated with local variations that are preserved over time by the force imposed by culture.

There are, for example, clonal variants of the potato that can number in the thousands in Peru. Each one is different, and each one is associated with a cultural pattern of use, and necessarily with the people who preserve it.

Animals

In animals, pure lines are associated with so-called breeds. In the dog, for example, the races define certain cultural patterns and relationships with humans.

The purer a breed is in animals, however, the greater the likelihood of suffering from genetic conditions.

In the process of maintaining the purity of certain traits, it has been selected for the homozygosity of other characters that are not beneficial for the survival of the individual and the species.

Genetic purity, however, conspires against genetic variability and diversity, which is what genetic improvement feeds on to proceed with selection.

Pure lines in other contexts

When a social construction is imposed on a biological fact, the manifestations in the real world are really disastrous.

This is how, in search of a biological impossibility, and in the name of a purity built socially on erroneous concepts, the human being has committed crimes of horrendous nature.

Eugenics, ethnic cleansing, racism and state segregation, the extermination of some and the supremacy of other particular human groups are born from a misconception of purity and inheritance.

Unfortunately, situations will be found in which attempts are made to justify these crimes with biological “arguments”. But the truth of the matter is that, biologically, the closest thing to genetic purity is clonality.

Is it a genetically pure clone?

However, scientific evidence suggests that this is not true either. In a bacterial colony, for example, which may contain about 10 ⁹ "clonal" individuals, the probability of finding a mutant for a single gene is practically equal to 1.

Escherichia coli, for example, has no fewer than 4,500 genes. If this probability is the same for all genes, it is most likely that the individuals in that colony are not all genetically the same.

Somaclonal variation, on the other hand, explains why this is also not true in plants with vegetative (clonal) modes of reproduction.

References

1. Birke, L., Hubbard, R., editors (1995) Reinventing Biology: respect for life and the creation of knowledge (race, gender and science). Indiana University Pres, Bloomington, IN.
2. Brooker, RJ (2017). Genetics: Analysis and Principles. McGraw-Hill Higher Education, New York, NY, USA.
3. Goodenough, UW (1984) Genetics. WB Saunders Co. Ltd, Pkiladelphia, PA, USA.
4. Griffiths, AJF, Wessler, R., Carroll, SB, Doebley, J. (2015). An Introduction to Genetic Analysis (11 ^th ed.). New York: WH Freeman, New York, NY, USA.
5. Yan, G., Liu, H., Wang, H., Lu, Z., Wang, Y., Mullan, D., Hamblin, J., Liu, C. (2017) Accelerated generation of selfed pure line plants for gene identification and crop breeding. Frontiers in Plant Science, 24: 1786. doi: 10.3389 / fpls.2017.01786.