A gene is the fundamental unit of heredity, containing the instructions that determine an organism’s specific traits inherited from parents. Think of an organism’s entire genetic makeup as a vast cookbook, with each gene representing a single recipe. These recipes dictate everything from appearance to the internal workings of the body. All plants and animals have genes, which are passed down from one generation to the next.
Where Genes Are Located
Every human body is composed of trillions of cells, and inside most of these cells is a specialized compartment called the nucleus. The nucleus acts as the control center, housing the genetic material. This material is organized into structures known as chromosomes. Humans typically have 46 chromosomes in 23 pairs in most of their cells. One chromosome from each pair is inherited from each parent.
Each chromosome is made of a long, tightly coiled molecule called deoxyribonucleic acid, or DNA. A gene is a specific segment of this long DNA molecule. To visualize this, if a cell is a library and chromosomes are bookshelves, the long strands of DNA are the books. A gene would be a specific chapter within one of those books, containing a precise set of instructions.
The complete set of genes in an organism is called its genome. The Human Genome Project mapped the entire human genome, estimating that humans have between 20,000 and 25,000 genes. More recent studies refined this to approximately 19,900 protein-coding genes. Although nearly every cell in the body contains a complete copy of the genome, each cell type only uses the genes relevant to its specific functions.
What Genes Actually Do
The primary function of a gene is to hold the instructions for building a specific molecule, most often a protein. Proteins are the workhorses of the cell, building everything from bones and hair to muscles and blood. They are responsible for the body’s growth, proper function, and overall health.
This process involves two main steps. First, the cell “reads” the gene’s DNA sequence and creates a temporary copy in the form of a molecule called messenger RNA (mRNA). This initial step is known as transcription. The mRNA molecule then carries these instructions from the nucleus out into the main part of the cell.
Once the mRNA arrives, the second step, called translation, begins. The cell’s machinery reads the mRNA’s code, which uses three-letter “words” called codons. Each codon specifies which amino acid to add to the growing protein chain. The cell then assembles a protein, which folds into a specific three-dimensional shape to perform its task.
How Genes Determine Traits
The observable characteristics of an organism, such as eye color, hair texture, and height, are known as its traits. These are the direct result of the proteins produced by genes. For instance, the gene for hair color provides instructions to produce melanin, the protein that gives hair its pigment. The specific type and amount of melanin produced determine a person’s hair color.
For most genes, an individual inherits two copies, one from each parent. These copies are not always identical. Different versions of the same gene are called alleles. For example, the gene that determines eye color has several alleles, including one for brown eyes and one for blue eyes. These small differences in the DNA sequence of alleles contribute to the unique physical features of each person.
The way these alleles interact determines the final trait. Some alleles are dominant, meaning only one copy is needed for the trait to be expressed, while others are recessive, requiring two copies. In the case of eye color, the allele for brown eyes is dominant over the recessive blue-eye allele. This means a person with one of each allele will have brown eyes, as the dominant allele masks the effect of the recessive one.
The Impact of Gene Changes
A change in the DNA sequence of a gene is called a mutation. These alterations can arise spontaneously when a cell divides or be caused by environmental factors. A mutation can be thought of as a typo in a recipe. If the typo is insignificant, the mutation is neutral, having no observable effect.
Sometimes, a mutation can be beneficial. It might lead to an improved version of a protein, offering a survival advantage. This is a driving force behind evolution, allowing organisms to adapt to changing environments. Over long periods, the accumulation of beneficial mutations can lead to new traits and even new species.
However, some mutations can be harmful. A harmful mutation can lead to a protein that does not function correctly or at all, which can result in a genetic disorder. For example, cystic fibrosis is caused by mutations in the CFTR gene. This gene provides instructions for a protein that regulates the flow of salt and fluids in and out of cells; when this protein is faulty, it leads to the condition’s symptoms.