Almost all traits have some variation among individuals of a species. To understand how this occurs, scientists first try to describe or measure the extent of the trait’s variation.
Sometimes, it’s most accurate to describe a trait as being discrete, meaning that there are a small number of possible variations, making it easy to assign an individual to a particular group. For example, you might observe that individuals of a particular plant species have either white or red flowers, but never any color in between. Here, when you organize your ear cards into two groups (with either free or attached earlobes), you’re treating that variation as a discrete trait.
Other traits are more accurately described as continuous, meaning that there are many, if not infinite, possible variations. For example, people can be almost any height or weight, and it would not be easy to decide who would count as being tall and who would count as being short in a group of people. Here, when you organize your ear cards into a continuous spectrum, you’re describing earlobe shape as a continuous trait, ranging from free to attached.
Scientists usually consider continuous traits to be complex traits. Complex traits are controlled by multiple genes and alleles, often with additional effects from environmental factors. Because many genes and alleles are involved, it’s difficult to precisely predict the inheritance pattern of a complex trait. The child of two parents of different heights, for example, may be closer in height to one parent than the other, be taller than the tallest parent or shorter than the shortest, or somewhere in between. Height depends on the precise combination of alleles a child inherits, and the environmental conditions under which the child develops.
Discrete traits often turn out to be simple traits, which means that they’re controlled by a single gene, and follow what's known as Mendelian patterns of inheritance. It’s easier to predict the inheritance pattern of a simple trait, as there are only a limited number of outcomes with known probabilities.
While earlobe shape has often been described in biology textbooks as being a discrete, simple trait, you probably had a hard time deciding where to draw the line between free and attached. If you decided that the continuous spectrum of earlobe shape was a more accurate way to describe the variation, you aren’t alone. Family and genetic studies show that earlobe attachment is actually not a simple trait, but rather a complex trait, affected by multiple genes and environmental factors.