One of the most common definitions of intelligence is the ability to learn and apply one's knowledge (Howard Gardner's Multiple Intelligences). Humans use their brains and intelligence every day, and the human ability to think and create is what allows us to be the dominant species on Earth. However, there has always been a gap in intelligence levels within our species. There are geniuses and they can solve complex mathematical equations without hesitation. On the other hand, we have a large percentage of people who have learning difficulties, such as mental retardation. Scientists have found that there are many factors that influence an individual's intelligence, both positively and negatively. Many connections have been made between intelligence, genetics, and biology, but many scientists have found that the environment in which someone is raised also plays a significant role in intelligence. In the 21st century, what causes high or low intelligence has not yet been fully determined, and scientists and researchers are discussing many theories and ideas. Your DNA is made up of hundreds and thousands of gene sequences that make you you. Your parents contributed to your genetic makeup, so half of your genes come from your mother and the other half from your father. Many scientists and researchers have established that approximately 50% of intelligence is heritable (Causes of Intelligence). The genes you inherited from your parents give clues to how you are built. Therefore, the structure of the brain can differ depending on the genes. Your genes determine how your nerves are enclosed in myelin. Myelin is a fatty insulator in the brain used to transmit nerve signals (more evidence than intelligence). The thicker the myelin, the faster your brain works... middle of paper... Genes, but where? Association for Psychological Science, October 2, 2012. Web. May 21, 2014. Mental retardation. Advameg, 2014. Web. May 21, 2014. More evidence that intelligence is largely inherited: Researchers find that genes determine the brain's processing speed. Science Daily, March 18, 2009. Web. May 14, 2014. Scientists "spot" intelligence. BBC News and Web. 14 May 2014. .The 3% Solution. The Economist newspaper, 10 May 2014. Web. 21 May 2014. .