What is the difference between dominant vs. recessive?

Editor’s note, 3/30/2012:
The examples listed in this article are traditionally taught in classes as simple dominant/recessive traits. Upon closer study, it turns out none of these traits are truly simple dominant/recessive! Where available, I have included links to answers that discuss this in more detail for each specific trait. In addition, please click here for more information about other traits not discussed here. 

Good question. Remember that for most genes, you have two copies of each gene that you inherited from your mother and your father.

Each copy of the gene could be different. For example one copy may give you blue eyes while another may give you brown.

So, what color are your eyes if you have both the brown and blue eye version of the eye color gene? Brown. This is where the idea of dominant and recessive comes in.

Dominant means that one of the versions trumps the other. In our example here, brown is dominant over blue so you end up with brown eyes.

The way people write out dominant and recessive traits is the dominant one gets a capital letter and the recessive one a lower case letter. So for eye color, brown is B and blue is b.

As I said above, people have two versions of each gene so you can be BB, Bb, or bb--BB and Bb have brown eyes, bb, blue eyes. Versions of genes are often dominant because the recessive version actually does nothing.

In the eye color example above, the brown version of the gene makes a pigment that turns your eye brown but the blue version does not make a blue pigment. Instead, it makes no pigment and an eye without pigment is blue.

As you can probably guess, if the blue version of the eye color gene made a pigment, then you'd get some mix of brown and blue. There are some cases like this for people. One of the easiest to understand is hair.

(2012 reality check: it’s a myth that eye color is a simple dominant/recessive trait.)

Curly hair

There are two "hair type" genes, curly and straight. If you have two copies of the curly version, you have curly hair and if you have two copies of straight hair version, you have straight hair.

What kind of hair do you have if you have a copy of each? Wavy.

Each of these versions contributes something so that you get a mixture of the two. You would write this out as CC is curly, SS is straight and CS is wavy.

In terms of what to talk about in your class, the hair type example I discussed above is a pretty good one for incomplete dominance. Maybe ask the class what kind of hair they have and what genes that means they have.

You can also ask them about their parent's hair type and whether their results fit the model.

(2012 reality check: curly hair isn’t caused by a single gene, so it’s not a perfect example of incomplete dominance.)

Bent pinky

The dominant version of the gene causes distal segment of pinky finger to bend distinctly inward toward the ring (fourth) finger.

(2012 reality check: it’s a myth that pinky shape is a simple dominant/recessive trait.)

Mid-digital hair

People lacking hair in the middle segments of the fingers have two recessive versions of the gene.

Tongue rolling

People with a dominant allele can roll their tongues into a tube shape. People with two recessive versions are non-rollers and can not learn to roll their tongues.

(2012 reality check: it’s a myth that tongue rolling is a simple dominant/recessive trait.)

Ear lobes

Recessives have attached ear lobes. People with a dominant version of the gene have detached ear lobes.

(2012 reality check: it’s a myth that earlobe attachment is a simple dominant/recessive trait. We have a newer article on this here.)

Thumb crossing

In a relaxed interlocking of fingers, left thumb over right results from having 1 or 2 copies of the dominant version of the gene. People with 2 recessives place right thumb over left.

(2012 reality check: it’s a myth that thumb crossing is a simple dominant/recessive trait.)