HOMOZYGOUS IN PROKARYOTES AND EUKARYOTES - BIOLOGY - 2025

A homozygous in genetics is an individual who has two copies of the same allele (the same version of a gene) at one or more loci (place on the chromosome). The term is sometimes applied to larger genetic entities such as whole chromosomes; in this context, a homozygous is an individual with two identical copies of the same chromosome.

The word homozygous is composed of two elements etymologically. The terms are homo -equal or identical- and zygote -fertilized ovum or the first cell of an individual originated through sexual reproduction.

A homozygous has the same type of allele for each gene on each homologous chromosome

Cell classification: prokaryotes and eukaryotes

Organisms are classified based on various properties associated with the genetic material (DNA) contained in their cells. Considering the cellular structure where the genetic material is located, organisms have been classified into two main types: prokaryotes (pro: before; karyon: nucleus) and eukaryotes (eu: true; karyon: nucleus).

Prokaryotes

In prokaryotic organisms the genetic material is confined to a particular region in the cytoplasm of cells called the nucleoid. The model organisms in this group correspond to bacteria of the Escherichia coli species, which have a single circular DNA chain, that is, their ends are joined together.

This chain is known as a chromosome and in E. coli it contains around 1.3 million base pairs. There are some exceptions to this pattern within the group, for example, some bacterial genera have straight-chain chromosomes such as spirochetes of the genus Borrelia.

The linear size or length of bacterial genomes / chromosomes is generally in the range of millimeters, that is, they are several times larger than the size of the cells themselves.

Genetic material is stored in a packaged form to reduce the space occupied by this large molecule. This packing is achieved through supercoiling, a twist on the main axis of the molecule that produces small twists that cause spinning.

In turn, larger twists of these small threads on themselves and the rest of the chain, thus reducing the distance and the space occupied between different sections of the circular chromosome and taking it to a condensed (folded) shape.

Eukaryotes

In eukaryotes the genetic material is located within a specialized compartment surrounded by a membrane; This compartment is known as the nucleus of the cell.

The genetic material contained within the nucleus is structured under a principle similar to that of prokaryotes, the supercoiling.

However, the degrees / levels of kinking are higher since the amount of DNA to accommodate is much higher. In eukaryotes the nucleus does not contain a single DNA chain or chromosome, it contains several of them and these are not circular, but linear and must be arranged.

Each chromosome varies in size depending on the species but they are usually larger than those of prokaryotes when compared individually.

For example, human chromosome 1 is 7.3 centimeters long, while the E. coli chromosome is approximately 1.6 millimeters long. For further reference, the human genome contains 6.6 × 10 ⁹ nucleotides.

Ploidy and chromosomes

There is another classification of organisms based on the amount of genetic material they contain, known as ploidy.

Organisms with a single set or copy of chromosomes are known as haploids (bacteria or reproductive cells in humans), with two sets / copies of chromosomes are known as diploids (Homo sapiens, Mus musculus, among many others), with four sets / Chromosome copies are known as tetraploids (Odontophrinus americanus, plants of the genus Brassicca).

Organisms with large numbers of chromosome sets are collectively known as polyploids. In many cases the extra sets of chromosomes are copies of a basic set.

For several years it was considered that characteristics such as ploidy greater than one were typical of organisms with a defined cell nucleus, but recent findings have shown that some prokaryotes have multiple chromosomal copies raising their ploidy, as demonstrated by the cases of Deinococcus radiodurans and Bacillus meagateriium.

Homozygous and dominance

In diploid organisms (such as the peas studied by Mendel) the two genes at a loci, or alleles, are inherited, one maternally and one parentally, and the allele pair together represents the genotype of that specific gene.

An individual who presents a homozygous (homozygous) genotype for a gene is one who possesses two identical variants or alleles at a given locus.

Homozygos, in turn, can be sub-classified into two types based on their relationship and contribution to the phenotype: dominant and recessive. It should be noted that both expressions are phenotypic properties.

Dominance

Dominant allele A

Dominance in the genetic context is a relationship between alleles of a gene in which the phenotypic contribution of one allele is masked by the contribution of the other allele of the same locus; in this case the first allele is recessive and the second is dominant (heterozygous).

Dominance is not inherited in the alleles or in the phenotype they produce, it is a relationship that is established based on the alleles present and can be modified by external agents such as other alleles.

A classic example of dominance and its relationship with the phenotype is the production of a functional protein by the dominant allele that finally produces the physical trait, while the recessive allele does not produce said protein in a functional form (mutant) and therefore does not contributes to the phenotype.

Dominant homozygous

Thus, a homozygous dominant individual for a trait / characteristic is one that possesses a genotype that presents two identical copies of the dominant allele (pure line).

It is also possible to find dominance in genotypes where the two dominant alleles are not found, but one dominant allele is present and one is recessive, but this is not a case of homozygosity, it is a case of heterozygosity.

In genetic analysis, dominant alleles are represented by a capital letter related to the trait being described.

In the case of the petals of the pea flower, the wild trait (in this case the purple color) is dominant and the genotype is represented as "P / P", denoting both the dominant trait and the homozygous condition, that is,, the presence of two identical alleles in a diploid organism.

Recessive homozygous

Recessive aa

On the other hand, an individual homozygous recessive for a particular trait carries two copies of the allele that codes for the recessive trait.

Continuing with the example of the pea, the recessive trait in the petals is white, so in individuals with flowers of this color, each allele is represented with a lowercase letter implying recessivity and the two identical recessive copies, so the genotype is symbolized as "p / p".

In some cases geneticists use a capital letter symbolically to represent the wild-type allele (for example P) and thereby symbolize and refer to a specific nucleotide sequence.

On the other hand, when a lowercase letter, p, is used, it represents a recessive allele that can be any of the possible types (mutations).

Dominant and recessive mutations

The processes by which a particular genotype is capable of producing a phenotype in organisms are varied and complex. Recessive mutations generally inactivate the affected gene and lead to a loss of function.

This can happen through a partial or complete removal of the gene, by the interruption of the expression of the gene or by the alteration of the structure of the encoded protein that finally alters its function.

On the other hand, dominant mutations often produce a gain of function, can increase the activity of a given gene product or confer a new activity to said product, and therefore can also produce inappropriate spatio-temporal expression.

This type of mutations can also be associated with loss of function, there are some cases where two copies of a gene are required for normal function so that the removal of a single copy can lead to a mutant phenotype.

These genes are known as haplo-insufficient. In some other cases the mutation can lead to structural changes in the proteins that interfere with the function of the wild-type protein encoded by the other allele. These are known as dominant negative mutations.

Recessive phenotypes in humans

In humans, examples of known recessive phenotypes are albinism, cystic fibrosis, and phenylketonuria. All of these are medical conditions with similar genetic bases.

Taking the latter as an example, individuals with this disease have a "p / p" genotype, and since the individual has both recessive alleles, it is homozygous.

In this case the "p" is related to the English term phenylketonuria and is lowercase to represent the recessive character of the allele. The disease is caused by abnormal processing of phenylalanine, which in normal conditions should be converted to tyrosine (both molecules are amino acids) by the enzyme phenylalanine hydroxylase.

Mutations in the vicinity of the active site of this enzyme prevent it from being able to bind to phenylalanine for later processing.

As a consequence, phenylalanine accumulates in the body and is converted into phenylpyruvic acid, a compound that interferes with the development of the nervous system. These conditions are collectively known as autosomal recessive disorders.

Homozygous and

The inheritance patterns and therefore the presence of alleles for a gene, both dominant and recessive, in the genotypes of individuals within a population obey Mendel's first law.

Mendel's law first

This law is known as the law of equal segregation of alleles and has molecular bases that are explained during the formation of gametes.

In diploid organisms that reproduce sexually, there are two main cell types: somatic cells and sex cells or gametes.

Somatic cells have two copies of each chromosome (diploid) and each of the chromosomes (chromatids) contains one of the two alleles.

Gametic cells are produced by germ tissues through meiosis where diploid cells undergo nuclear division accompanied by a chromosomal reduction during this process, consequently they only present one set of chromosomes, therefore they are haploid.

Meiosis

During meiosis the achromatic spindle is anchored to the centromeres of the chromosomes and the chromatids are separated (and therefore the alleles as well) towards opposite poles of the stem cell, producing two separate daughter cells or gametes.

If the individual producing the gametes is homozygous (A / A or a / a) then the total number of gametic cells produced by him will carry identical alleles (A or a, respectively).

If the individual is heterozygous (A / a or a / A) then half of the gametes will carry one allele (A) and the other half the other (a). When sexual reproduction is complete, a new zygote is formed, the male and female gametes fuse to form a new diploid cell and a new pair of chromosomes and therefore alleles is established.

This process originates a new genotype that is determined by the alleles contributed by the male gamete and the female gamete.

In Mendelian genetics, homozygous and heterozygous phenotypes do not have the same probabilities of appearing in a population, however, the possible allelic combinations associated with the phenotypes can be inferred or determined through genetic cross analysis.

If both parents are homozygous for a gene of the dominant type (A / A), then the gametes of both will be of type A in their entirety and their union will invariably result in an A / A genotype.

If both parents have a homozygous recessive genotype (a / a), then the offspring will invariably result in a homozygous recessive genotype as well.

Population genetics and evolution

In evolutionary theory, it is said that the engine of evolution is change and at the genetic level change occurs through mutations and recombinations.

Mutations often involve changes in some nucleotide base of a gene, although they can be of more than one base.

Most mutations are considered spontaneous events associated with the error rate or fidelity of polymerases during DNA transcription and replication.

There is also much evidence of physical phenomena causing mutations at the genetic level. For its part, recombinations can produce exchanges of entire sections of chromosomes but are only associated with events of cell duplication, such as mitosis and meiosis.

In fact, they are considered a basic mechanism to generate genotypic variability during gamete formation. Incorporation of genetic variability is a hallmark of sexual reproduction.

Genes and evolution

Focused on genes, it is currently considered that the engine of inheritance and therefore evolution, are the genes that present more than one allele.

Those genes that only have one allele can hardly cause an evolutionary change if all individuals in the population have two copies of the same allele as exemplified above.

This is because as genetic information is passed from one generation to another, changes will hardly be found in that population unless there are forces that produce variations in genes such as those mentioned above as well.

The simplest evolutionary models are those that only consider one locus and their objective is to try to predict the genotype frequencies in the next generation, from the data of the existing generation.

References

1. Ridley, M. (2004). Evolutionary Genetics. In Evolution (pp. 95-222). Blackwell Science Ltd.
2. Griswold, A. (2008) Genome packaging in prokaryotes: the circular chromosome of E. coli. Nature Education 1 (1): 57
3. Dickerson RE, Drew HR, Conner BN, Wing RM, Fratini AV, Kopka, ML The anatomy of A-, B-, and Z-DNA. 1982. Science, 216: 475-485.
4. Iwasa, J., Marshall, W. (2016). Control of Gene Expression. In Karp's Cell and Molecular Biology, Concepts And Experiments. 8th Edition, Wiley.
5. Hartl DL, Jones EW (2005). Genetics: Analysis of Genes and Genomes. pp 854. Jones & Bartlett Learning.
6. Mendell, JE, Clements, KD, Choat JH, Angert, ERExtreme polyploidy in a large bacterium. 2008. PNAS 105 (18) 6730-6734.
7. Lobo, I. & Shaw, K. (2008) Thomas Hunt Morgan, genetic recombination, and gene mapping. Nature Education 1 (1): 205
8. O'Connor, C. (2008) Chromosome segregation in mitosis: The role of centromeres. Nature Education 1 (1): 28
9. Griffiths AJF, Wessler, SR, Lewontin, RC, Gelbart, WM, Suzuki, DT, Miller, JH (2005). An Introduction to Genetic Analysis. (pp. 706). WH Freeman and Company.
10. Lodish, HF (2013). Molecular cell biology. New York: WH Freeman and Co.