Introduction

Every cell in your body contains an enormous amount of genetic information stored in DNA. However, not all of this DNA has the same purpose. Small sections of DNA, called genes, contain the instructions that tell cells how to build specific proteins. These proteins determine many of an organism's characteristics and enable cells to carry out their everyday functions.

Genes are often described as the instructions for life because they guide growth, development, reproduction, and the countless chemical processes that keep organisms alive. Although every species is unique, many genes are surprisingly similar across different organisms, showing that all life on Earth shares a common evolutionary history.

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What Is a Gene?

A gene is a section of DNA that contains the instructions for making a specific protein or controlling a particular characteristic (trait).

Genes are the basic units of heredity.

Each gene contains information that cells use to produce proteins needed for growth, development, and normal body functions.

A chromosome contains thousands of genes, each with its own specific instructions.

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Figure 1. Genes are sections of DNA found on chromosomes.

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Where Are Genes Found?

Genes are located on chromosomes, which are found inside the nucleus of most plant and animal cells.

The organisation is:

• Cell
• Nucleus
• Chromosome
• DNA
• Gene

Each chromosome carries many different genes.

Humans normally have:

• 46 chromosomes
• About 20,000–21,000 genes

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How Do Genes Influence Characteristics?

Genes influence characteristics by providing instructions for making proteins.

Proteins perform many important jobs, including:

• Building tissues.
• Carrying oxygen.
• Fighting disease.
• Speeding up chemical reactions as enzymes.
• Sending signals between cells.

The proteins produced by genes help determine many inherited characteristics.

For example:

• Genes influence eye colour by controlling the production of pigments.
• Genes influence hair texture by controlling proteins found in hair.
• Genes influence blood groups by determining which molecules appear on the surface of red blood cells.

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Figure 2. Genes influence traits by providing instructions for making proteins.

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Genes and Proteins

The relationship between genes and proteins can be summarised as:

Gene → Protein → Trait

The process works like this:

1. A gene contains the instructions.
2. The cell reads those instructions.
3. A protein is produced.
4. The protein performs a specific function or contributes to a characteristic.

For example:

A gene may contain instructions for producing a protein that forms part of muscle tissue. That protein helps muscles grow, repair themselves, and function properly.

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Traits Controlled by Genes

A trait is a characteristic of an organism.

Many traits are strongly influenced by genes.

Examples include:

• Eye colour.
• Natural hair colour.
• Blood group.
• Earlobe attachment.
• Flower colour in plants.
• Fur colour in animals.

Some traits are controlled mainly by genes, while others are influenced by both genes and the environment.

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Figure 3. Many inherited traits are controlled by genes.

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Genes and the Environment

Not every characteristic is determined only by genes.

Some traits are influenced by both:

• Genes
• Environmental factors

Examples include:

Genes provide the potential, while the environment can influence how that potential is expressed.

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Why Different Organisms Share Many Genes

Although living organisms look very different, many of their genes are remarkably similar.

For example:

• Humans and chimpanzees share about 98–99% of their DNA.
• Humans and mice share many genes involved in growth and development.
• Plants, animals, and fungi all share genes responsible for basic cellular processes.

This is because all living organisms evolved from common ancestors over millions of years.

Many essential biological processes are the same across different species.

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Figure 4. Many organisms share similar genes because they evolved from common ancestors.

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Why Shared Genes Matter

Because many genes are shared:

• Scientists can study mice to learn about human diseases.
• Medicines developed for animals may help humans.
• Crops can be improved using knowledge of plant genetics.
• Evolutionary relationships between species can be investigated.

Shared genes help scientists understand how life has evolved and how organisms function.

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Genes and Cell Specialisation

Almost every cell in your body contains the same DNA.

However, different cells use different genes.

For example:

• Muscle cells produce proteins needed for movement.
• Nerve cells produce proteins for transmitting signals.
• Skin cells produce protective proteins.

By switching different genes on or off, cells become specialised for different jobs.

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Figure 5. Different cell types use different genes to produce the proteins needed for their specialised functions.

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Why Genes Are Important

Genes allow organisms to:

• Grow.
• Develop.
• Repair tissues.
• Reproduce.
• Pass characteristics to offspring.
• Adapt and evolve over generations.

Without genes, cells would have no instructions for making the proteins needed for life.

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Worked Example

Question

Complete the sequence.

Gene → __________ → Trait

Solution

Gene → Protein → Trait

Genes contain instructions for making proteins, and proteins help produce observable characteristics.

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Real-World Connection

Scientists can identify genes associated with certain inherited diseases. This allows doctors to diagnose some genetic conditions, estimate the risk of passing them to future generations, and, in some cases, develop treatments that target the proteins produced by those genes. Understanding gene function is therefore essential in modern medicine.

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Did You Know?

Humans share many of their genes not only with other mammals but also with much simpler organisms. For example, humans share a large number of genes with fruit flies and even yeast. Because these organisms use many of the same genes to control basic cellular processes, scientists often study them to learn more about human biology and disease.

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Key Terms

Chromosome – A tightly coiled structure made of DNA and proteins that contains many genes.

DNA (Deoxyribonucleic acid) – The molecule that stores genetic information.

Gene – A section of DNA that contains the instructions for making a specific protein or controlling a trait.

Heredity – The passing of genetic information from parents to offspring.

Protein – A molecule built from amino acids that performs structural and functional roles in living organisms.

Trait (Characteristic) – A feature or quality of an organism that may be influenced by genes.

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Key Takeaways

• A gene is a section of DNA that contains instructions for making a specific protein or influencing a particular trait.
• Genes influence characteristics by directing the production of proteins, which carry out many functions in cells.
• The relationship can be summarised as Gene → Protein → Trait.
• Many inherited characteristics, such as eye colour, blood group, and natural hair colour, are controlled by genes.
• Different cell types use different genes, allowing them to perform specialised functions.
• Many different organisms share similar genes because they evolved from common ancestors, reflecting the shared biology of life on Earth.