Genetics

Genetics is the scientific study of heredity and variation in living organisms. At its foundation, genetics examines how traits are transmitted from parents to offspring and how these traits can change over time.

The Fundamental Unit: DNA

To understand genetics from first principles, we must begin with deoxyribonucleic acid (DNA). DNA serves as the molecular blueprint for life, containing the instructions necessary to build and maintain living organisms. This molecule consists of four chemical bases: adenine (A), thymine (T), guanine (G), and cytosine (C). These bases pair specifically—A with T, and G with C—forming the famous double helix structure.

The sequence of these bases creates a chemical code, much like letters forming words in a language. Different sequences encode different instructions, and these instructions determine the characteristics of living things.

From DNA to Genes

Genes represent specific segments of DNA that contain instructions for making proteins or regulating other genes. Each gene occupies a particular location on a chromosome and serves as a functional unit of heredity. Humans possess approximately 20,000 to 25,000 genes distributed across 23 pairs of chromosomes.

Genes function through a two-step process: transcription and translation. During transcription, the DNA sequence is copied into messenger RNA (mRNA). During translation, this mRNA is read by cellular machinery called ribosomes, which assemble amino acids into proteins according to the genetic instructions.

Inheritance Patterns

Genetic inheritance follows predictable patterns that Gregor Mendel first described in the 19th century. Each individual inherits two copies of each gene—one from each parent. These gene variants are called alleles. The combination of alleles determines whether a trait will be expressed and how it will appear.

Some alleles are dominant, meaning they will be expressed even if only one copy is present. Others are recessive, requiring two copies for expression. This principle explains why children may exhibit traits that neither parent displays but that grandparents possessed.

Variation and Mutation

Genetic variation arises through several mechanisms. Sexual reproduction creates new combinations of existing genes through the shuffling process that occurs during meiosis. Additionally, mutations—changes in the DNA sequence—introduce entirely new genetic variants into populations.

Most mutations have neutral or negative effects, but occasionally they provide advantages that natural selection can favor. This process drives evolution and explains the remarkable diversity of life on Earth.

The Central Dogma

Francis Crick articulated the central dogma of molecular biology, which describes the flow of genetic information: DNA makes RNA, and RNA makes proteins. This unidirectional flow of information from genes to proteins underlies all biological processes and connects the molecular level of genetics to the observable traits we see in living organisms.

This framework provides the foundation for understanding how genetic information is stored, transmitted, and expressed, forming the basis for all modern genetic research and applications in medicine, agriculture, and biotechnology.