Dominant and recessive traits

Here are two parents, a mother and a father. Both have brown eyes. Here are their four children. Three kids with brown eyes, and one with... blue eyes!

Can parents with brown eyes have children with blue eyes? Is that possible? Yes, it is in fact possible. We'll see shortly how it works. Genes for eye colour come in pairs, just like all genes.

All people have one trait from the mother, and one from the father. Let's say the traits are these coloured circles. If the trait for blue eyes comes from both parents, the child will have blue eyes. If the trait for brown eyes comes from both parents, the child will have brown eyes. And if one parent gives the trait for brown eyes, and the other for blue, then the child will have...

brown eyes! The trait for brown eyes takes over. It is a dominant trait. The trait for blue eyes holds back if it meets with a more dominant trait. It is a recessive trait.

Let's go back to the parents we just saw. Both of them have both traits: for brown eyes, and for blue eyes. The coloured circles show the traits. They each have both traits, but both have brown eyes, because the trait for brown eyes is dominant. Every child gets one trait from the father.

It can be either the trait for brown eyes... Or the one for blue. And then one trait comes from the mother. In the same way it can be either the trait for brown... ... or for blue eyes.

We see that there are four ways the children can inherit their genes. Children born with this set of genes get the trait for brown eyes from both parents. This child gets brown eyes. In this box, and this one, we find children who have inherited both traits. Since the trait for brown eyes is dominant, children with this mix of genes will get brown eyes.

But here. Here the trait for blue eyes has come from both parents. This trait is recessive, but since the dominant trait is not present, the recessive genes will have effect, and the children's eyes will be blue. This is a genetic diagram. It describes how traits distribute in a new generation.

It shows... ... how two parents with brown eyes can get both brown eyed, and blue eyed kids. ... and it shows the probability of it. Three out of four kids, three quarters, will get brown eyes, and one quarter will get blue eyes. Pause the film if you need to think about how it works.

Let's make another genetic diagram. This mother has blue eyes. Which means she got the trait for blue eyes from both her parents. It must be that way, since the trait for blue eyes is recessive. The father is brown-eyed, and he got that trait from both his parents.

The circles in brown and blue represent the traits. The genetic diagram is made the same way. The children get one of the traits from the mother... ... and one of the traits from the father, and it can happen in four different ways. In all boxes, it looks the same.

Both the trait for blue eyes, and the trait for brown. All children will have similar sets of genes. And all children will have brown eyes, since the trait for brown eyes is dominant, and the trait for blue eyes is recessive. There are many variations of genetic diagram. What does the diagram look like if the father carries only the trait for blue eyes, and the mother carries both traits?

Try drawing a genetic diagram yourself, to represent this. So it is fully possible that two parents with brown eyes get blue eyed children. And we have seen how it works with the help of our genetic diagrams of dominant and recessive traits. Our example with eye colour is a simplification. Eye colour, and many other characteristics, are in fact determined by several different traits in a complex interplay.

But the example can help you to understand how dominant and recessive traits work.