Homozygous Recessive: Definition & Disorders

Homozygous Recessive: Definition & Disorders
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  • 0:00 What is Homozygous Recessive?
  • 0:35 An Example
  • 1:20 What Makes a Gene Recessive?
  • 2:20 Diseases
  • 3:35 Lesson Summary
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Lesson Transcript
Instructor: Beth Skwarecki
Many disorders are homozygous recessive, from cystic fibrosis to albinism. Learn how a person (or a rat) can end up with two recessive genes and why this can cause genetic disorders.

What is Homozygous Recessive?

Most plants and most animals - including you, if you're human - have two copies of each of their genes. If both copies are identical, we say that individual is homozygous for that gene.

A recessive gene is one that has to be homozygous to have an effect on the plant's or animal's traits. If you have a lab rat with a dominant gene, you'll see its effect whether the rat has one copy of the gene or two. But if you're looking for the effects of a recessive gene, you'll need a rat that is homozygous recessive, or who carries two identical copies of the recessive gene.

Examples of Homozygous Recessive Gene

Let's use our humble lab rat as an example. Wild rats are typically brown, but most lab rats are albino, meaning they don't have any pigment in their fur or eyes. That's why they appear white.

The albinism gene lives at a certain locus, or place on the DNA strand. There are two versions of the gene, called alleles. Let's call the dominant allele C, for Color. That means we would use a lowercase c to represent the recessive albino allele.

A rat can be brown if it has two color alleles or if it has one color allele and one albino allele. That's why we say the color allele is dominant. Meanwhile, our albino lab rats have two of the albino alleles. They are homozygous for the recessive allele.

What Makes a Gene Recessive?

Usually, a recessive gene is one that is broken. In the case of albinism, the C gene codes for a protein that helps make melanin, the pigment that makes rats brown. A rat with one copy of the gene is still able to make melanin, but if the rat is homozygous for the broken version, he can't make melanin at all.

We inherit one copy of a gene from each parent, so if a male and female rat are both albino, their offspring will all inherit one albino from Mom and one albino from Dad. If both parents are albino and color mixed, some offspring could be color, some could be albino, and some could be mixed, just like their parents.

Some recessive genes can cause severe or fatal diseases. If these genes were dominant, the individuals who carried them would die, and the genes would be lost from the gene pool. Recessive disorders survive because they can be passed on by healthy carriers who are heterozygous.


Here are a few examples of genetic disorders that occur when somebody is homozygous for a recessive gene.

Cystic fibrosis is one example of a homozygous recessive disease. The broken gene is involved in transporting chloride, something all cells need to do. Symptoms include mucus in the lungs and difficulty breathing, as well as damage to the pancreas and other organs. People with cystic fibrosis have shortened lifespans and usually cannot have children, so the disorder is passed on by healthy carriers.

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