What is Heterozygous? - Definition, Traits & Example

What is Heterozygous? - Definition, Traits & Example
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  • 0:01 Genes
  • 0:33 Example of a Heterozygote
  • 1:27 Codominance
  • 3:00 Lethal Gene Mutations
  • 3:34 Lesson Summary
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Lesson Transcript
Instructor: Beth Skwarecki
We all have two copies of each of our genes. If your two copies are slightly different than each other, congratulations, you're heterozygous! Learn how heterozygosity occurs, and why it can be a good thing to be slightly mismatched.


Genes often come in different versions, which are called alleles. For example, a gene that makes the brown pigment in your eyes might come in a regular (brown) version and a mutant (blue) version. Because you have two copies of your DNA (one from Mom and one from Dad), you might end up with two different alleles of the same gene. In that case, we would say you are heterozygous. The opposite of that is homozygous, two identical alleles.


Let's take dogs as an example: friendly Labrador retrievers. Your pet Lab, let's call him Rover, has black fur, although members of his doggy family include chocolate-brown relatives as well as black ones.

In many cases, one allele is dominant over another. In this case, black is dominant over chocolate. That means chocolate is recessive. Rover's mom had two black alleles, so she had black fur. His dad had two chocolate alleles, so he had chocolate fur. Rover inherited one black allele from his mother and one chocolate allele from his father, so he is heterozygous: he has one of each gene.

Because the chocolate color is recessive, heterozygotes like Rover have black fur. But Rover is a carrier for the chocolate allele. If he has puppies of his own, he could pass on the chocolate gene to them.


Not all traits follow this dominant/recessive pattern. Some are codominant, meaning that a heterozygote has a phenotype that is different from homozygotes. A phenotype can include the physical characteristics, behaviors, traits and development of an organism.

One example of codominance is the Rex gene in rats. Wild rats have a regular coat of fur, but the Rex mutation can make rats hairless. You won't see these rats in your local sewer, but you might see them in a pet store or - I am not making this up - a rat show. Think dog show, but with impeccably groomed rats in little cages.

  • A rat that is homozygous for the wild type allele will have normal fur.
  • A rat that is homozygous for the Rex allele will be hairless, because the gene causes hair to weaken and curl so much it breaks off.
  • But a rat that is heterozygous for the Rex allele doesn't have normal fur. Heterozygotes have just a little curl in their fur, and it doesn't break off. That means that a Rex heterozygote is a curly-haired rat!

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