Difference Between Allele and Locus

Main Difference – Allele vs Locus

Allele and locus are two terms used to describe the properties of a particular gene on a chromosome. The main difference between allele and locus is that allele is an alternative form of a gene whereas locus is the position of an allele in the chromosome. Alleles arise by mutations and can be found in the same locus of the homologous chromosome pair. Homologous chromosomes pair during the metaphase 1 of the meiosis 1. Genetic recombination occurs between alleles in the same locus by chromosomal crossover. This may lead to the creation of new allele combinations in the gametes. Ultimately, the new allele combinations bring genetic variation within a population.

Key Areas Covered

1. What is an Allele
      – Definition, Characteristics, Role
2. What is a Locus
      – Definition, Characteristics, Role
3. What are the Similarities Between Allele and Locus
      – Outline of Common Features
4. What is the Difference Between Allele and Locus
      – Comparison of Key Differences

Key Terms: Allele, Dominant, G-banding, Gene, Heterozygous, Homologous Chromosomes, Homozygous, Locus, Recessive

What is an Allele

An allele is an alternative form of a gene. Alleles arise by mutations. They are present in the same locus of the homologous chromosome pair. An allele determines a specific trait of a particular organism. These traits pass from one generation to the other by sexual reproduction.

The process of allele transmission was first described by the law of segregation by Gregor Mendel in 1865. The diploid organisms present two alleles of the same gene since they contain two sets of chromosomes. This allele pair can contain either the same nucleotide sequence (homozygous) or different nucleotide sequences (heterozygous). In the heterozygous condition, only one allele is expressed, and the other is in the repressed form. The expressing allele is called the dominant allele, and the repressed allele is called the recessive allele. The complete masking of the recessive allele by the dominant allele is called the complete dominance. Complete dominance is a type of Mendelian inheritance.

Figure 1: Different Eye Colors

The non-Mendelian inheritance patterns include incomplete dominance, codominance, multiple alleles, and polygenic traits. In incomplete dominance, both alleles in the heterozygous pair are expressed. In codominance, a mixed phenotype of both alleles in the heterozygous allele pair can be observed. Multiple alleles are the presence of more than two alleles in the population to determine a particular trait. In polygenic traits, the phenotype is determined by many genes. Skin color, eye color, height, weight, and hair color of humans are polygenic traits. Four different eye colors of humans are shown in figure 1.

What is a Locus

The term locus refers to the location of a gene or a particular nucleotide sequence on a chromosome. The locus of a particular gene is determined by gene mapping. All loci in a particular genome are called the genetic map of that genome. The components of the label of a locus may contain the number of the chromosome and the chromosome arm to which the locus belongs and the number of the nearby bands of the chromosome. Humans contain 23 pairs of homologous chromosomes. Each homologous chromosome contains same genes at the same loci. Most chromosomes consist of two arms, which are connected by the centromere. The long arm of the chromosome is called the q arm, and the short arm is called the p arm. Giemsa banding or G-banding is the technique used to stain the condensed regions of the chromosomes.

Figure 2: Different Loci on the Human Chromosome 11

The G-banding technique can be used to identify chromosomes since each chromosome exhibit a unique banding pattern with the Giemsa stain. The less condensed areas that are not stained by the Giemsa stain are the active genes. For example, the gene OCA1 is located on the 11q1.4-a2.1. This means the gene is located on the long arm of the chromosome 11 and between the sub-band4 of band 1to the sub-band 1 of band 2. The ends of the chromosomes or the telomeres are labeled as ‘ptel’ and ‘qtel’. For example, the end of the long arm of the chromosome 2 is labeled as 2qtel. The different loci of the human chromosome 11 are shown in figure 2.

Similarity Between Allele and Locus

Both allele and locus are two terms used to describe the properties of a gene on the chromosomes.

Definition

Allele: An allele is an alternative form of a gene that is arisen by mutations and is found in the same locus of the homologous chromosomes.

Locus: A locus refers to the position of an allele on the chromosome.

Significance

Allele: Allele is a nucleotide sequence of a gene.

Locus: Locus serves as a marker.

Number

Allele: A gene may contain two or more alleles.

Locus: One locus of the homologous chromosome pair may contain one or two alleles.

Responsible for

Allele: Alleles are responsible for variable traits within a population.

Locus: Different loci consist of different alleles.

Examples

Allele: I^A, I^B, and i are the three alleles that determine the human blood type.

Locus: The 11q1.4-a2.1 is the locus of the OCA1 gene in the human genome.

Conclusion

Allele and locus are two different terms used to describe the properties of a gene. An allele is an alternative form of a gene. Different alleles can arise due to the mutations of a gene. Locus is the position of a gene on the chromosome. The alleles of a particular gene can be found in the same loci of the homologous chromosome pair. An allele describes a nucleotide sequence of a gene while a locus describes the position of that allele on the chromosome. This is the difference between allele and locus.

Reference:

1. Bailey, Regina. “How Alleles Determine Traits in Genetics.” ThoughtCo, Available here. Accessed 23 Aug. 2017.
2.“Locus (Genetics).” Familypedia, Available here. Accessed 23 Aug. 2017.

Image Courtesy:

1. “Deep Blue eye” By Look Into My Eyes – originally posted in Flickr (CC BY 2.0) via Commons Wikimedia
2. “Beautiful green eye” By Look Into My Eyes – Flickr: emerald green eye (CC BY 2.0) via Commons Wikimedia
3. “Hazel eye1” By Yoldasso – Own work (CC BY-SA 3.0) via Commons Wikimedia
4. “Lens5” By Cecikierk – Own work (Public Domain) via Commons Wikimedia
5. “Human chromosome 11 – 550 bphs” By National Center for Biotechnology Information, U.S. National Library of Medicine (Public Domain) via Commons Wikimedia