Allozyme: Definition & Electrophoresis

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson, we will discuss how scientists differentiate between several species that look very similar. We will focus on allozymes, which scientists use as a marker of species relatedness.

Not All Species Look Different

Did you know that there are over 3,000 species of mosquito? They all look about the same to us, right? Sometimes, when scientists look at closely related organisms, they will look exactly the same. Only complex behavioral analysis, or genetic tricks of the trade, can be used to tell whether two organisms are the same species or not. So it could be true that there are many more species of mosquito that we don't know about yet!

Let's look at allozymes, one way of determining genetic relatedness in species. To do that, let's back up for a bit and talk about enzymes.


Enzymes are proteins in an organism's body that help chemical reactions go faster. They're vitally important for all sorts of reasons. Enzymes help you break down sugars so you can use the energy contained within the sugar. Enzymes also help you store sugars for later, by helping to build a storage molecule called glycogen. Enzymes help blood clot, and so on and so forth. Without your enzymes, you'd be dead.

Scientists can also look at enzymes to determine two organisms' relatedness to each other.


An allozyme is a form of an enzyme that differs from a closely related enzyme, but differs only a little bit. An allozyme differs by a single allele (alternative form of the same gene) at a single locus (location on the gene).

Why do we care about allozymes? Well, these tiny differences, though they might not change the function of an enzyme, can tell us a lot about the evolutionary history of the organism they inhabit. These tiny differences come from mutations, or random changes in the molecular sequence of our DNA. Though the changes themselves are random, they occur at a relatively predictable rate. Thus, there would be fewer of these changes between you and a closely related species than between you and a more distantly related species.

Allozymes can therefore be used to map family trees between different species within the same genus (the taxonomic rank above species). Scientists have used allozymes to map out closely related species in plants and animals, in all sorts of groups.

So how do scientists determine these tiny differences in allozymes, when we can't even see them? One tool that scientists use to look at allozymes is capillary electrophoresis. Electrophoresis separates protein fragments (or other substances, such as DNA fragments) by size. Basically, you put some enzymes on a sort of racetrack made of a jellylike substance called an agarose gel. You then run an electrical current through the gel to make the enzymes move. The smaller the enzymes are, and the more highly charged they are, the faster they travel. You can then determine the relative size and charge of the fragments by seeing which fragments went the farthest. So, when two enzymes are similar but not exactly the same, these small differences can be teased out by electrophoresis.


Practical Uses of Allozymes

The disease malaria is transmitted by mosquitoes, so believe it or not, a lot of scientists are really interested in mosquitoes! Two species of mosquito that transmit malaria are Anopheles gambiae and Anopheles arabiensis. They look exactly the same, even to mosquito scientists. However, scientists can tell them apart using allozyme analysis.

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