Understanding Alleles in Biology: Definition and Examples

Alleles are a fundamental concept in genetics, representing different variations of a gene located at the same locus on a chromosome. This article explores the definition of alleles and provides detailed examples to illustrate their significance in biological research and genetic studies.

Definition of Alleles

The term allele originates from the Greek word allos, meaning 'other.' It was first coined by geneticists to describe alternative forms or variations of a gene. In simpler terms, alleles are distinct versions of a gene that control a specific trait or characteristic.

Alleles in Genetic Studies

A well-known example of alleles is found in Gregor Mendel's pea plant experiments. Mendel studied seven distinct characteristics, such as plant height (tall vs. short), flower color (white vs. violet), and seed pod color (green vs. yellow). These variations are expressed through different alleles of the genes influencing these traits.

Genetic Instructions and Alleles

An organism’s genetic code acts as an instruction manual for building its physical traits. These instructions are organized into genes, which can be thought of as chapters in this manual. For instance, the gene responsible for eye color or the ability to digest lactose as an adult is a chapter in this manual.

While most individuals within a population share the same genes, the difference lies in the specific versions of these genes. This means that even though the table of contents and chapter headers are the same, the actual content can vary. Different alleles can lead to variations in an individual's traits. For example, one person might have an allele that instructs their body to produce blood type A, while another might have an allele for blood type B.

Example of Alleles in Insulin Production

To further illustrate the concept of alleles, consider the gene responsible for insulin production in humans. This gene is located on chromosome 11, and its sequence varies between different individuals, leading to different versions of the gene, or alleles.

Human Gene (Chromosome 11)

atg gcc ctg tgg atg cgc ctc ctg ccc ctg ctg gcg ctg ctg gcc ctc tgg gga cct gac
ncca gcc gca gcc ttt gtg aac caa cac ctg tgc ggc tca cac ctg gtg gaa gct ctc tac
ncta gtg tgc ggg gaa cga ggc ttc ttc tac aca ccc aag acc cgc cgg gag gca gag gac
nctg cag gtg ggg cag gtg gag ctg ggc ggg ggc cct ggt gca ggc agc ctg cag ccc ttg
ngcc ctg gag ggg tcc ctg cag aag cgt ggc att gtg gaa caa tgc tgt acc agc atc tgc
ntcntcc ctc tac cag ctg gag aac tac tgc aac tag

Lowland Gorilla Gene (Chromosome 11)

atg gcc ctg tgg atg cgc ctc ctg ccc ctg ctg gcg ctg ctg gcc ctc tgg gga cct gac
ncca gcc gcg gcc ttt gtg aac caa cac ctg tgc ggc tcc cac ctg gtg gaa gct ctc tac
ncta gtg tgc ggg gaa cga ggc ttc ttc tac aca ccc aag acc cgc cgg gag gca gag gac
nctg cag gtg ggg cag gtg gag ctg ggc ggg ggc cct ggt gca ggc agc ctg cag ccc ttg
ngcc ctg gag ggg tcc ctg cag aag cgt ggc atc gtg gaa cag tgc tgt acc agc atc tgc
ntcntcc ctc tac cag ctg gag aac tac tgc aac tag

The genes for insulin production in humans and lowland gorillas are almost identical, with only slight differences in specific base pairs. In this example, the differences are highlighted in bold. Despite these minor variations in the third base of codons, the amino acid sequence of the insulin precursor remains the same, demonstrating how these genetic variations can be functionally neutral.