Genetics is the scientific field dedicated to understanding heredity, exploring how living organisms acquire characteristics from their parents and how traits are passed down through generations. To grasp how biological information is encoded and expressed, it is important to understand foundational terms like genes and alleles.
What is a Gene?
A gene is the fundamental unit of heredity in living organisms. It is a specific segment of deoxyribonucleic acid (DNA) that carries instructions for building and maintaining an organism. These segments are organized along chromosomes, which are thread-like structures found within the nucleus of cells. Genes direct the synthesis of specific proteins or functional ribonucleic acid (RNA) molecules.
The precise sequence of nucleotides within a gene dictates the order of amino acids in a protein. Genes are responsible for the diverse traits an organism exhibits, such as eye color, height, or susceptibility to certain conditions. They act as informational units that determine an organism’s characteristics.
What is an Allele?
An allele is a specific variant form of a gene. While a gene determines a trait, alleles represent different versions of that trait. For instance, a gene might dictate flower color, but various alleles could lead to red, white, or pink flowers. These variations arise from slight differences in the DNA sequence at a specific location on a chromosome.
These sequence differences can be as minor as a single nucleotide change. Such alterations can influence how a protein functions or how much of it is produced, leading to observable differences in an organism’s traits. Each gene has multiple alleles within a population, contributing to the genetic diversity seen among individuals.
How Genes and Alleles Interact
A gene establishes a general characteristic, such as hair color, while alleles provide the specific instructions for that characteristic, like brown, black, or blonde hair. Organisms that reproduce sexually inherit two alleles for each gene, receiving one from each parent. These two alleles can be identical or different.
When an individual inherits two identical alleles for a gene, they are homozygous for that trait. If an individual inherits two different alleles for a gene, they are heterozygous. In heterozygous individuals, the interaction between the two alleles determines the observable trait, known as the phenotype. This often involves dominance, where one allele’s trait is expressed while the other’s is masked.
For example, in human eye color, the gene has various alleles. An allele for brown eyes is dominant over an allele for blue eyes. An individual with one brown-eye allele and one blue-eye allele will have brown eyes. This illustrates how genes provide the framework for traits, and alleles deliver the specific instructions that result in an organism’s characteristics.