You might know someone who was born with a genetic disorder, such as cystic fibrosis or Down Syndrome. And you might have wondered how someone inherits these types of disorders. It all goes back to Mendel! Mendel’s rules laid the foundation for understanding the genetics of all organisms, including humans. We can apply Mendel’s rules to describe how many human traits and genetic disorders are inherited. Some disorders are caused by a recessive allele, while other disorders are caused by a single dominant allele. Therefore, we can draw a Punnett square to predict the number of offspring that may be affected with these diseases, just like we predicted for other traits in the previous lessons. Since Mendel’s time, we have also expanded our knowledge of inheritance and understand that genes are located on chromosomes. Now we can now explain special inheritance patterns that don’t fit Mendel’s rules.