- Dominance and recessivity
- Example of dominance and recessivity
- Mutant alleles
- Codominance
- ABO
- Haploids and diploids
- References
The alleles are different versions of a gene and can be dominant or recessive. Each human cell has two copies of each chromosome, having two versions of each gene.
Dominant alleles are the version of the gene that is phenotypically expressed even with a single copy of the gene (heterozygous). For example, the allele for black eyes is dominant; a single copy of the gene for black eyes is needed to express itself phenotypically (that the person at birth has eyes of that color).
Recessive alleles aa expressed in the white butterfly. The brown butterfly has a dominant allele (A); you only need one copy to express that gene
If both alleles are dominant, it is called codominance. For example with blood type AB.
Recessive alleles only show their effect if the organism has two copies of the same allele (homozygous). For example, the gene for blue eyes is recessive; it takes two copies of the same gene for it to be expressed (for the person to be born with blue eyes).
Dominance and recessivity
The qualities of dominance and recessiveness of the alleles are established based on their interaction, that is, one allele is dominant over another depending on the pair of alleles in question and the interaction of their products.
There is no universal mechanism by which dominant and recessive alleles act. Dominant alleles do not physically "dominate" or "repress" recessive alleles. Whether an allele is dominant or recessive depends on the particularities of the proteins they encode.
Historically, dominant and recessive patterns of inheritance were observed before the molecular basis of DNA and genes, or how genes encode the proteins that specify traits, were understood.
In that context, the terms dominant and recessive can be confusing when it comes to understanding how a gene specifies a trait; however, they are useful concepts when it comes to predicting the probability that an individual will inherit certain phenotypes, especially genetic disorders.
Example of dominance and recessivity
There are also cases in which some alleles may present both dominance and recessive characteristics.
The allele of hemoglobin, called Hbs, is an example of this, since it has more than one phenotypic consequence:
Individuals homozygous (Hbs / Hbs) for this allele have sickle cell anemia, a hereditary disease that causes pain and damage to organs and muscles.
Heterozygous individuals (Hbs / Hba) do not present the disease, therefore, Hbs is recessive for sickle cell anemia.
However, heterozygous individuals are much more resistant to malaria (a parasitic disease with pseudo-flu symptoms) than homozygos (Hba / Hba), giving the Hbs allele dominance for this disease.
Mutant alleles
A recessive mutant individual is one whose two alleles must be identical for the mutant phenotype to be observed. In other words, the individual must be homozygous for the mutant allele in order for it to display the mutant phenotype.
In contrast, the phenotypic consequences of a dominant mutant allele can be observed in heterozygous individuals, carrying one dominant allele and one recessive allele, and in homozygous dominant individuals.
This information is essential to know the function of the affected gene and the nature of the mutation. Mutations that produce recessive alleles usually result in gene inactivations leading to a partial or complete loss of function.
Such mutations can interfere with the expression of the gene or alter the structure of the protein encoded by the latter, altering its function accordingly.
For their part, the dominant alleles are generally the consequence of a mutation that causes a gain in function. Such mutations can increase the activity of the protein encoded by the gene, change the function, or lead to an inappropriate spatio-temporal pattern of expression, thereby conferring the dominant phenotype in the individual.
However, in certain genes, dominant mutations can lead to loss of function as well. There are cases known as haplo-insufficiency, so called because the presence of both alleles is necessary to present a normal function.
The removal or inactivation of just one of the genes or alleles can produce a mutant phenotype. In other cases, a dominant mutation in one allele can lead to a structural change in the protein it codes for and this interferes with the function of the protein of the other allele.
These mutations are known as dominant-negative and produce a phenotype similar to that of mutations that cause loss of function.
Codominance
Codominance is formally defined as the expression of the different phenotypes normally displayed by the two alleles in a heterozygous individual.
That is, an individual with a heterozygous genotype composed of two different alleles can show the phenotype associated with one allele, the other, or both at the same time.
ABO
The ABO system of blood groups in humans is an example of this phenomenon, this system is made up of three alleles. The three alleles interact in different ways to produce the four blood types that make up this system.
the three alleles are i, Ia, Ib; an individual can possess only two of these three alleles or two copies of one of them. The three homozygous i / i, Ia / Ia, Ib / Ib, produce phenotypes O, A and B respectively. Heterozygotes i / Ia, i / Ib, and Ia / Ib produce genotypes A, B and AB respectively.
In this system, alleles determine the shape and presence of an antigen on the cell surface of red blood cells that can be recognized by the immune system.
While alleles e Ia and Ib produce two different forms of the antigen, allele i does not produce antigen, therefore, in genotypes i / Ia and i / Ib alleles Ia and Ib are completely dominant over allele i.
On the other hand, in the Ia / Ib genotype, each of the alleles produces its own form of antigen and both are expressed on the cell surface. This is known as codominance.
Haploids and diploids
A fundamental genetic difference between wild and experimental organisms is in the number of chromosomes that their cells carry.
Those that carry only one set of chromosomes are known as haploids, while those that carry two sets of chromosomes are known as diploids.
Most complex multicellular organisms are diploid (such as the fly, mouse, human, and some yeasts such as Saccharomyces cerevisiae, for example), while most simple single-celled organisms are haploid (bacteria, algae, protozoa, and sometimes S. cerevisiae too!).
This difference is fundamental because most genetic analyzes are carried out in a diploid context, that is, with organisms with two chromosomal copies, including yeasts such as S. cerevisiae in its diploid version.
In the case of diploid organisms, many different alleles of the same gene can occur among individuals in the same population. However, since individuals have the property of having two sets of chromosomes in each somatic cell, an individual can carry only one pair of alleles, one on each chromosome.
An individual who carries two different alleles of the same gene is a heterozygote; an individual who carries two equal alleles of a gene is known as homozygous.
References
- Ridley, M. (2004). Evolutionary Genetics. In Evolution (pp. 95-222). Blackwell Science Ltd.
- Lodish, HF (2013). Molecular cell biology. New York: WH Freeman and Co.
- Griffiths AJF, Wessler, SR, Lewontin, RC, Gelbart, WM, Suzuki, DT, Miller, JH (2005). An Introduction to Genetic Analysis. (pp. 706). WH Freeman and Company.
- Genetic Science Learning Center. (2016, March 1) What are Dominant and Recessive ?. Retrieved March 30, 2018, from
- Griswold, A. (2008) Genome packaging in prokaryotes: the circular chromosome of E. coli. Nature Education 1 (1): 57
- Iwasa, J., Marshall, W. (2016). Control of Gene Expression. In Karp's Cell and Molecular Biology, Concepts And Experiments. 8th Edition, Wiley.
- O'Connor, C. (2008) Chromosome segregation in mitosis: The role of centromeres. Nature Education 1 (1): 28
- Hartl DL, Jones EW (2005). Genetics: Analysis of Genes and Genomes. pp 854. Jones & Bartlett Learning.
- Lobo, I. & Shaw, K. (2008) Thomas Hunt Morgan, genetic recombination, and gene mapping. Nature Education 1 (1): 205