Genetics is the study of heredity — how traits are passed from parents to offspring. DNA (deoxyribonucleic acid) is the molecule that stores genetic information in the form of a double helix.

Genes are segments of DNA that code for specific traits (like eye color or blood type). Humans have about 20,000–25,000 genes organized on 23 pairs of chromosomes. One chromosome in each pair comes from each parent.

Traits can be dominant or recessive. A dominant allele (represented by a capital letter, like B) is expressed when present. A recessive allele (lowercase, like b) is only expressed when two copies are present (bb). A Punnett square helps predict the probability of offspring inheriting specific traits.

Evolution through natural selection explains how populations change over time. Organisms with traits better suited to their environment are more likely to survive and reproduce, passing those traits to the next generation.