Did someone ever tell you that you have your mom's eyes? Your dad's nose, maybe? You may notice that some of your physical and behavioral features are quite similar to those of your parents. How do children inherit characteristics of their parents? The answer is simple: genetics. To be more specific, the genes for many traits are passed down in families from parents to children. A basic principle of genetics contributes to this: the principle of "segregation."

The principle of "segregation" works for both plants and animals alike. Since everyone comes from two parents, they inherit half of their traits from their mother and half of their traits from their father. Therefore you will have two "alleles," or for each gene, one from each parent. Because of this, your body "expresses" that gene.

However, there is one problem with this principle. What if there are two alleles that...