Genetics, combined with a person's environment and experiences influence their development and behavior. You may have heard of the age-old "nature versus nurture" debate. Scientists study how much of what makes us "us" is genetic.
Humans knew for ages that living things inherited characteristics from their parents. Because of these observations, they used "selective breeding" to improve plants and animals.
Genetic disorders are a problem caused by abnormalities in the genes. Some genetic disorders are hereditary. This means we are more susceptible to them because of our family history. Other disorders are due to mutations in our DNA. DNA mutations are part of evolution, but sometimes they can cause abnormalities.
Some disorders are common like Familial Hypercholesterolemia, and some are rarer like Hemophilia.