Allele Definition and Examples


Allele Definition

An allele is one of two or more versions of a gene that are found at the same place, or locus, on a chromosome. Genes, which consist of DNA, act as instructions to make molecules called proteins. Each person inherits two alleles for each gene (one from each parent). In many cases, different alleles result in different observable traits. Each allele is a unique sequence variant of the same genetic locus.

Allele Etymology

The term “allele” is a short form of the word “allelomorph,” which originates from the Greek word allelo– ( ‘of each other’) and morph (‘form’). English geneticists William Bateson and Edith Rebecca Saunders introduced the word. The term reflects the concept that each allele is one of a pair or series of different forms of a gene.

Importance of Alleles

Alleles play a crucial role in genetic diversity, which is essential for the survival and adaptation of species. They are the basis for hereditary variation, influencing everything from physical appearance to disease susceptibility. Understanding alleles is key in fields like genetics, evolutionary biology, and medical research.

Allele vs. Gene

While an allele refers to a variant form of a gene, a gene is a broader term that refers to a unit of heredity in general. A gene is a sequence of DNA that contains the information to produce a specific protein or set of proteins, whereas alleles are the different forms this gene might take.

Allele Location

Alleles for a given gene are always located at the same position, or locus, on homologous chromosomes. To clarify:

  1. Same Locus on Homologous Chromosomes: Each chromosome in a homologous pair has the same genes arranged in the same order, but the versions of these genes (alleles) can differ. For instance, one chromosome might carry an allele for blue eyes, and its homologous partner might carry an allele for brown eyes. Both alleles occur at the same genetic locus on each chromosome.
  2. Single Locus for Each Gene: Typically, a gene occupies a single locus on a chromosome. This means that a specific gene is found at a specific location on the chromosome. The different versions of that gene, or alleles, are found at the same locus on homologous chromosomes.
  3. Multiple Loci on a Chromosome: A single chromosome contains many genes, each at its own unique locus. This means there are multiple loci on a chromosome, but each locus typically corresponds to a single gene (or a specific part of a gene in the case of complex genes).

Characteristics of Alleles

  1. Variability: Alleles vary in their DNA sequence, which sometimes affects the function of the protein produced. The change can be as small as a variation of a single nucleotide, or a larger portion of DNA may differ. Duplications and deletions of DNA within a gene also produce different alleles.
  2. Dominance and Recessiveness: In a diploid organism, which has two copies of each chromosome, sometimes one allele is dominant over another. A dominant allele expresses itself when one copy is present, while a recessive allele requires two copies.
  3. Codominance and Incomplete Dominance: Some alleles exhibit codominance or incomplete dominance, where neither allele is completely dominant or recessive.

Allele Genotype vs Phenotype

Different alleles represent different genotypes or genetic sequences of a gene. But, different alleles do not always result in a different observable trait or phenotype. The relationship between alleles and phenotypes is complex. Whether a different allele leads to a different phenotype depends on several factors:

  1. Dominance and Recessiveness: In many cases, if an individual has one dominant allele and one recessive allele for a trait, the dominant allele masks the effect of the recessive allele. This leads to the expression of only the dominant trait. In such scenarios, different alleles (dominant and recessive) do not result in a different phenotype because the presence of the dominant allele determines the phenotype.
  2. Codominance and Incomplete Dominance: In codominance, both alleles have equal expression, producing a phenotype that includes both traits. In incomplete dominance, the phenotype is a blend or an intermediate of the two alleles.
  3. Penetrance and Expressivity: Penetrance refers to the proportion of individuals with a particular genotype that actually express the associated phenotype. Expressivity is the degree to which a trait is expressed. Even with the same allele, the phenotype is not always be expressed (complete penetrance) or may vary in its expression (variable expressivity).
  4. Modifier Genes and Environmental Factors: Other genes (modifier genes) influence the expression of a trait. Additionally, environmental factors significantly impact how or whether a gene is expressed. Thus, the same allele in different environments or genetic backgrounds sometimes results in different phenotypes.
  5. Silent Mutations: Some alleles differ in their DNA sequence without causing any change in the function of the protein they encode. These are often called silent mutations and usually do not result in a different phenotype.

Dominant and Recessive Phenotypes

Alleles determine whether a phenotype, or physical trait, is dominant or recessive. A dominant allele is expressed even if only one copy is present, while a recessive allele requires two copies (homozygous) for expression.

Examples of Alleles

  1. Blood Type: The ABO blood group system demonstrates codominance (A and B alleles) and recessiveness (O allele).
  2. Eye Color: Various alleles contribute to eye color, with brown typically dominant over blue.
  3. Cystic Fibrosis: A recessive allele causes cystic fibrosis. A person must inherit two copies of the mutated gene to manifest the disease.
  4. Sickle Cell Anemia: Sickle cell anemia results from an allele of the hemoglobin gene. Individuals with one copy of the allele (heterozygous) resist malaria, demonstrating a survival advantage in certain environments.

Epialleles

Epialleles are a unique type of allele where the variation in trait expression comes from epigenetic modifications rather than by differences in DNA sequence. Epigenetics involves changes in gene expression that do not alter the DNA sequence but still pass from one generation to the next. These modifications usually occur in the form of DNA methylation or histone modification.

  1. DNA Methylation: Involves adding a methyl group to the DNA, typically at cytosine bases. Methylation changes the activity of a DNA segment without changing the sequence. When methylation occurs in a gene promoter, it typically represses gene transcription.
  2. Histone Modification: Histones are proteins around which DNA is wrapped. Modifications to histones (such as acetylation or methylation) influence gene expression by altering how tightly the DNA winds around the histones, thereby affecting the accessibility of the DNA to transcription factors.

Epialleles influence traits such as flower color, fruit size, and disease resistance in plants. In humans, epigenetic changes affect diseases like cancer and the effects of aging. Importantly, environmental factors influence epigenetic changes.

Idiomorphs

Idiomorphs are a concept primarily in the study of fungal genetics. In certain fungi, there are distinctly different sequences (idiomorphs) at a given locus. These idiomorphs are so unlike each other that they do not appear to be alleles of the same gene.

Idiomorphs are especially significant in determining mating types in fungi. In animals and plant, sex chromosomes determine sex. However, in many fungi, the specific idiomorphs present at one or more loci determine sexual identity.

References

  • Daxinger, Lucia; Whitelaw, Emma (2012). “Understanding transgenerational epigenetic inheritance via the gametes in mammals”. Nature Reviews Genetics. 13 (3): 153–62. doi:10.1038/nrg3188
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  • Hartl, Daniel L.; Elizabeth W. Jones (2005). Essential Genetics: A Genomics Perspective (4th ed.). Jones & Bartlett Publishers. ISBN 978-0-7637-3527-2.
  • Ogasawara, K.; Bannai, M.; et al. (1996). “Extensive polymorphism of ABO blood group gene: three major lineages of the alleles for the common ABO phenotypes”. Human Genetics. 97 (6): 777–83. doi:10.1007/BF02346189
  • Smigielski, Elizabeth M.; Sirotkin, Karl; Ward, Minghong; Sherry, Stephen T. (2000). “dbSNP: a database of single nucleotide polymorphisms”. Nucleic Acids Research. 28 (1): 352–355. doi:10.1093/nar/28.1.352