Allele is a term used to describe the alternative form of a gene, which is a segment of DNA occupying a specific locus (location) on a chromosome.[1] In simpler words, it is one of two or more variants of the DNA sequence of a particular gene. These variants are produced because of modifications (commonly called mutations) in the DNA sequence. Alleles differ from each other by one or a few bases and can result in changes to the observable characteristics of an organism, called phenotype. The allele that is produced as a result of mutation is called mutant, while the allele that hasn’t undergone any mutation is called wild-type allele. The simplest alleles are the single nucleotide polymorphisms (SNP), but they can also be insertions and deletions of several base pairs, in some cases several thousands.[2]
Most organisms have two sets of chromosomes called diploid chromosomes. An individual usually inherits two alleles, one from each parent. Both alleles lie in a specific location on each chromosome. If both alleles are the same, it can be said that the individual is homozygous for that allele. Similarly, if the alleles inherited from both parents are different, the individual is heterozygous for that allele. Often, organisms in a population have more than two alleles at each locus on chromosomes among different individuals.
Type of Alleles
The interaction between two alleles at a locus can often be classified as either dominant or recessive, based on the resemblance of the phenotype of the heterozygote to that of the two homozygous genotypes. When an individual has two different alleles for a particular gene and the phenotype of the heterozygote is identical to that of one of the homozygotes, the allele that is expressed is referred to as dominant,[3] while the unexpressed allele is referred to as recessive. The degree and pattern of dominance can vary widely among loci. Certain disorders can arise due to the inheritance of two recessive alleles for a single-gene trait (characteristic) by an individual. Recessive genetic disorders are a group of genetic disorders that are inherited in a recessive manner. Some examples of recessive genetic disorders include cystic fibrosis, albinism, galactosemia, phenylketonuria (PKU), and Tay-Sachs disease.[4]Some other conditions are caused by recessive alleles located on the X chromosome, resulting in males being more commonly affected than females due to having only one copy of the X chromosome. Examples of genetic disorders include red-green color blindness and Fragile X syndrome.[5]
Differences between a gene and an allele
The terms "allele" and "gene" are frequently used interchangeably, despite having distinct meanings. A gene is defined as a region of DNA that contains the instructions required for the synthesis of a particular protein molecule. An allele, on the other hand, is one of the possible variations of a gene that can exist at a particular location, on a chromosome.[6] One easily recognizable example is a gene that controls eye color. There may be more than one allele for this gene, such as "blue," "brown," and "green." Each type is a different form of gene, which helps explain why people have different eye colors. Genes contain the genetic information of an organism, while alleles are the different versions or variants of a gene. Genes are responsible for encoding the blueprint for various traits, while alleles are responsible for introducing diversity and contributing to the different variations of those traits,[7] as well as for enabling genetic differentiation and identification of certain populations.
Explore more
- Single Nucleotide Polymorphisms (SNPs)
- DNA Basics Chapter 8: Genotypes and Phenotypes on the MyHeritage blog
- Genealogy Glossary: Common DNA Terms Explained on the MyHeritage Knowledge Base
References
- ↑ Graur, D (2016). Molecular and Genome Evolution. Sunderland MA (USA): Sinauer Associates, Inc.
- ↑ Elston, Robert; Satagopan, Jaya; Sun, Shuying (2012). "Genetic Terminology."Statistical Human Genetics. Methods in Molecular Biology (Clifton, N.J.). Vol. 850. pp. 1–9
- ↑ Hartl, Daniel L.; Elizabeth W. Jones (2005). Essential genetics: A genomics perspective (4th ed.). Jones & Bartlett Publishers. p. 600.
- ↑ "Autosomal Recessive: Cystic Fibrosis, Sickle Cell Anemia, Tay-Sachs Disease." University of Rochester Medical Center
- ↑ "What is Fragile X Syndrome?" Centers for Disease Control and Prevention
- ↑ "Difference Between Gene and Allele." Toppr
- ↑ "What Is Genetic Diversity and Why Does it Matter?"Frontiers.