The completion of the Human Genome Project heralded a new era in human genetic testing to predict individuals at risk from many common diseases. DNA markers can also be used to track one's ancestry. Eye diseases such as age-related macular degeneration and glaucoma have been important examples of the success of genome-wide association studies. Resource-strapped genetic services have been limited in providing DNA testing for many well-established hereditary diseases. Thus, several direct-to-consumer genetic services have arisen to fill the gap. However, there is a major need for research into interpreting the results of such tests of up to one million DNA markers. Studies of population, family and twins sharing common diseases help us clarify the significance of gene–disease associations. However, as identical twins show us, for some conditions our genes do not absolutely determine our destiny and environmental factors interact with our genetic profile.