Complementation Tests: Alleles, Crosses & Loci

Complementation Tests: Alleles, Crosses & Loci
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  • 0:04 Complements
  • 1:47 Complementation Tests
  • 3:16 Example Cross
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Lesson Transcript
Instructor: Kristin Klucevsek

Kristin has taught college Biology courses and has her doctorate in Biology.

Don't judge an organism by its phenotype; sometimes, two organisms will have the same phenotype but completely different genotypes. In this lesson, we'll learn how a complementation test can identify if mutations occurred within the same gene or different genes.

Complements

Today's lesson will be full of complements. You might be used to getting a compliment - 'Your hair looks lovely today!' 'Wow, you smell fabulous!' 'That is a beautiful shade of neon pink you are wearing!'

But today's lesson isn't exactly about those kinds of compliments. In genetics, genes can complement each other, too - but notice the spelling difference. Genes complement each other with an 'e.' To understand what this means, and that it isn't exactly one gene telling another that its nucleotides are in a really unique pattern, we have to go back to a basic concept in genetics.

Remember that for a diploid organism, there are two copies of each gene in all non-gamete cells. These two copies could be the same or they could be different. We call different versions of the same gene alleles.

On any given chromosome, there are tons of genes. In the flying hamster, there is a gene we'll call B, and that controls coat color. The dominant allele of this gene leads to a brown hamster, while the recessive allele would lead to a white hamster.

Let's say that one of these gene copies, or alleles, is wildtype in one hamster - meaning it has no mutation. We can represent this allele by a capital B. This allele contributes to a brown coat color in flying hamsters.

Let's say the other copy of this gene, or the other allele, has a recessive mutation. We can represent this allele by a lowercase b. Because this mutation is recessive, the phenotype you see in this organism will be wildtype (the hamster is brown), even though the organism carries a mutation. In genetics, we say that the wildtype allele (capital B) then complements, or makes up for, the mutation (little b).

Complementation Tests

We just happen to know a researcher who normally studies the gene for coat color in flying hamsters. He usually studies the gene B. He knows white hamsters with a mutation in this gene usually have two recessive copies of this gene, or a bb genotype.

Our researcher happens to come across two pure-breeding flying hamsters with the same phenotype - they are both white. The term pure-breeding means that these hamsters only produce white hamster offspring when they are mated with each other, so they are homozygous for the same genetic mutation. These two white hamsters get plenty of complements from their fellow brown hamsters on their unique coat color.

Sure, these beauties have the same phenotype, but do these two hamsters have the same genotype? Are mutations within the same gene causing this phenotype or are the mutations within different genes?

To answer this question, we have to ask if these mutations can complement each other using a complementation test. A complementation test is a method that uses a cross to identify if two mutations are located within the same gene or different genes. This method is used if the same recessive phenotype is seen in an organism and you want to know whether or not a mutation in the same gene is responsible.

In our researcher's case, he has two white flying hamsters caused by recessive mutations, and he wants to know at what locus, or what gene location, these mutations exist. Are these two white hamsters caused by mutations in the gene B? Or is there another gene be involved?

Example Cross

A complementation test is better understood with an example. So, the first thing our researcher will do is cross the two pure-breeding white hamsters with the recessive phenotype, white coat color. If all the offspring from the cross are hamsters with white coat color, too, this means that the two original white hamsters all had mutations within this same gene, gene B.

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