- Definition of allele
- Allele location
- Discovery of alleles
- Allele types
- Recombination
- Allele frequency
- Why do the allele frequencies change?
- Alleles and diseases
- References
The alleles are different variants or alternative ways in which a gene may occur. Each allele can manifest as a different phenotype, such as eye color or blood group.
On chromosomes, genes are located in physical regions called loci. In organisms with two sets of chromosomes (diploids), the alleles are located at the same locus.
Brown eye color is related to a dominant allele. Source: pixabay.com
Alleles can be dominant or recessive, depending on their behavior in the heterozygous organism. If we are in a case of complete dominance, the dominant allele will be expressed in the phenotype, while the recessive allele will be obscured.
The study of allelic frequencies in populations has had a remarkable impact in the area of evolutionary biology.
Definition of allele
The genetic material is divided into genes, which are segments of DNA that determine phenotypic characteristics. By possessing two identical sets of chromosomes, diploid organisms possess two copies of each gene, called alleles, located in the same position of pairs of identical chromosomes, or homologous.
Alleles often differ in the sequence of nitrogenous bases in DNA. Although small, these differences can produce obvious phenotypic differences. For example, they vary the color of the hair and eyes. They can even cause hereditary diseases to express themselves.
Allele location
A notable characteristic of plants and animals is sexual reproduction. This implies the production of female and male gametes. The female gametes are found in the ovules. In plants, male gametes are found in pollen. In animals, in sperm
Genetic material, or DNA, is found on chromosomes, which are elongated structures within cells.
Plants and animals have two or more identical sets of chromosomes, one of them from the male gamete and the other from the female gamete that gave rise to them through fertilization. Thus, alleles are found in DNA, inside the nucleus of cells.
Discovery of alleles
Around 1865, in an Austrian monastery, the monk Gregory Mendel (1822-1884), experimented with crosses of pea plants. By analyzing the proportions of plants with seeds of different characteristics, he discovered the three fundamental laws of genetic inheritance that bear his name.
In Mendel's day nothing was known about genes. Consequently, Mendel proposed that plants transmitted some kind of matter to their offspring. Today that "stuff" is known as alleles. Mendel's work went unnoticed until Hugo de Vries, a Dutch botanist, disclosed it in 1900.
Modern biology rests on three fundamental pillars. The first is the binomial nomenclature system of Carlos Linneo (1707-1778) proposed in his work Systema Naturae (1758). The second is the theory of evolution, by Carlos Darwin (1809-1892), proposed in his work The Origin of Species (1859). The second is the work of Mendel.
Allele types
Each pair of alleles represents a genotype. Genotypes are homozygous if both alleles are identical, and heterozygous if they are different. When the alleles are different, one of them may be dominant and the other recessive, with the phenotypic characteristics determined by the dominant one prevailing.
Variations in allele DNA do not necessarily translate into phenotypic changes. Alleles can also be codominant, both affecting the phenotype with equal intensity, but differently. Furthermore, a phenotypic characteristic can be affected by more than one pair of alleles.
Recombination
The appearance, in the next generation, of various genotypes, or combinations of alleles, is called recombination. By acting on a large number of genes, this process causes genetic variation, which allows each individual produced by sexual reproduction to be genetically unique.
The phenotypic variability caused by recombination is essential for plant and animal populations to adapt to their natural environment. This environment is variable both in space and time. Recombination ensures that there are always individuals well adapted to the conditions of each place and of the moment.
Allele frequency
The proportion of genotypes of a pair of alleles in a population is p 2 + 2 pq + q 2 = 1, where p 2 represents the fraction of individuals homozygous for the first allele, 2 pq the fraction of heterozygous individuals, and q 2 the fraction of homozygous individuals for the second allele. This mathematical expression is known as the Hardy-Weinberg law.
Why do the allele frequencies change?
In the light of population genetics, the definition of evolution implies the change of allele frequencies over time.
The frequency of alleles in a population changes from one generation to the next due to natural or random selection. This is known as microevolution. Long-term microevolution can lead to macroevolution or the appearance of new species. Random microevolution produces genetic drift.
In small populations, the frequency of an allele can increase or decrease from generation to generation by chance. If the change in one direction is repeated in successive generations, all members of a population can become homozygous for given alleles.
When a small number of individuals colonize a new territory, they carry with them a frequency of alleles that, by chance, may be different from that of the original population. This is known as the founder effect. Combined with genetic drift, it can lead to the loss or fixation of certain alleles just by chance.
Alleles and diseases
Albinism, cystic fibrosis, and phenylketonuria are due to having inherited two recessive alleles for the same gene. If the defective allele is on the X chromosome, as in the case of green color blindness and fragile X syndrome, the disease affects only the male sex.
Other diseases, such as pseudoachondroplastic dwarfism and Huntington's syndrome, occur when an individual inherits a dominant allele. That is, pathological conditions can present as dominant or recessive alleles.
References
- Edelson, E. 1999. Gregor Mendel and the roots of genetics. Oxford University Press, New York.
- Freeman, S., Herron, JC 1998. Evolutionary analysis. Pearson Prentice and Hall, Upper Saddle River, New Jersey.
- Griffiths, AJF, Suzuki, DT, Miller, JH, Lewontin, RC, Gelbart, WM 2000. An Introduction to Genetic Analysis. WH Freeman & Co., New York.
- Hapgood, F. 1979. Why males exist - an inquiry into the evolution of sex. William Morrow and Company, New York.
- Klug, WS, Cummings, MR, Spencer, CA 2006. Concepts of genetics. Pearson Prentice and Hall, Upper Saddle River, New Jersey.
- Mange, EJ, Mange, AP 1999. Basic human genetics. Sinauer Associates, Sunderland, Massachusetts.
- Mayr, E. 2001. What evolution is? Orion Books, London.
- Robinson, TR 2010. Genetics for dummies. Wiley, Hoboken, New Jersey.