Genetic variation is the diversity in DNA sequences found among individuals of the same species. This diversity is crucial for a species' survival because it helps organisms adapt to environmental changes. Genetic variation begins with fertilization, where an egg and sperm cell merge. Each of these cells carries 23 chromosomes, up to 46 in the fertilized egg. Chromosomes are long DNA strands that contain genes, the basic units of heredity.

Genes exist in different versions called alleles, which can influence various traits, such as hair and eye color, or even the risk of certain diseases. The specific mix of alleles inherited from both parents creates a unique set of genetic instructions, which shape the offspring's physical appearance and health characteristics, termed the phenotype.

The genotype is an individual's specific genetic makeup, determined by the alleles received from both parents. This genetic combination affects how traits are expressed. For instance, eye color is influenced by the alleles for eye color each parent contributes. If both parents provide a recessive allele for blue eyes, their child will likely have blue eyes. Conversely, if one allele is for brown eyes and the other for blue, the dominant brown allele usually prevails, resulting in brown eyes for the child.

The terms homozygous and heterozygous describe whether the two alleles for a particular gene are the same or different. Being homozygous means both alleles are identical, while heterozygous means they are different. This distinction can significantly impact an individual's health. For example, sickle cell anemia is a condition where individuals with two recessive alleles for the hemoglobin gene have the disease. However, those who are heterozygous, carrying one normal and one sickle cell allele, do not show symptoms of the disease but have a protective advantage against malaria, which is beneficial in areas where malaria is common.