A gene is considered to be the basic physical and functional block of heredity. Genes are made up of DNA. Some genes constitute molecules called proteins. However, most of the genes do not constitute proteins. Genes differ in size in humans and their range may vary from a few hundred DNA bases to more than 2 million bases. International research is conducted and it is found in Human Genome Project, that humans have genes between 20,000 and 25,000.

Genetic drift is a change in the frequency of a gene variation within a population over time. This change in the genotype or gene variation frequency should always be random for genetic drift to occur. Genetic Drift cannot occur due to the influences of the environment.

For example, a mother with black eyes and a blue-eyed father can have children with black or blue eyes. If black is the dominant genotype, even though there is a 50% chance of having blue eyes. There might be a possibility of having all children with black eyes.