-A middle school student from Illinois
May 27, 2015
Great question! The answer has to do with how different genes work to give traits. And how differences in genes affect those traits.
Before we go into the why, let’s back up and go over different kinds of traits. Then, we’ll review how genes cause traits. And then finally, we’ll tackle the why of your question!
There Are Different Kinds of Traits
Now let’s say that one parent plant has white flowers and another parent plant has red flowers. This means that the parent plant with white flowers is rr. And for reasons we’ll discuss below, the parent plant with red flower is RR. When these parents are paired, the Rr seeds make plants with pink flowers!
This is an example of incomplete dominance, when the inherited alleles of a trait blend. This happens when the dominant trait is not totally dominant, and the recessive trait is not totally recessive. It’s like mixing paint: red paint + white paint = pink paint.
Now let’s tackle codominant traits. For this situation we will use the example of blood type.
There are three alleles for blood type: A, B and O. For codominant traits, we will focus on blood types A and B. (If you want to learn about how type O fits into the mix, click here).
Imagine we have one parent with type A blood and the other with type B blood. To keep things simple, we will say that the parent with type A blood is AA and the other parent with type B blood is BB.