Understanding Animal Model Options and Specific Uses of Alleles in Genetic Research – What Is a Conditional Allele?
Genetically modified animals are the cornerstone of research programs across a tremendous range of topics. Mice in particular are invaluable because genetic modification techniques have been available for decades, so a large number of different modified mice are already available. However this means that the diverse options for genetically modified mice can be overwhelming. Here we’ll discuss one option that should be evaluated when choosing a mouse model: a conditional allele. What is a conditional allele and why are they so important for research? Read on to learn how a conditional allele can deliver more accurate control over genetic modifications.
Alleles and Conditional Alleles
Before going deeper into just what is a conditional allele and what it is used for, let’s take a step back to clarify what alleles are in the context of genetically modified animals. In simple terms, an allele is a variant form of a gene. The word allele could refer to the different gene sequences that exist naturally in a population, or in an animal model it could be a change that was deliberately introduced. Normally a gene’s sequence is the same in essentially every cell of the body. In the early days of genetically modified mice this was also true of genetic changes introduced by researchers. However certain research questions require an animal model where a gene has a different allele in different cells. For example it could be necessary for most cells to have a functional copy of a gene while just a few cells contain a non-functioning copy. Conditional alleles therefore are designed to enable control over exactly when and where different alleles will be expressed.
Comparing a Knockout Allele and a Conditional Knockout Allele
The simplest way to explain what is a conditional allele is to directly compare two related mouse model types, knockouts and conditional knockouts. Knockout mice are a type of mouse model where a specific gene has been altered in a way that permanently disrupts its function. This disruption, or knockout, affects the gene in every single cell of the mouse’s body and at all stages of its life. A common way to knock out a gene would be to delete part of its sequence – this is a permanent change that can’t be reversed. Such models have proven extremely beneficial for many studies but their limitations can get in the way of certain experiments. A conditional knockout allele can achieve the same result, disruption of the gene’s function, but with more control. This is because the conditional knockout allele will initially function just like the natural unmodified gene. Part of the gene can be deleted just like the knockout allele but the deletion has to be triggered by researchers. This principle applies to all conditional alleles: they initially express one sequence, then can be switched to expressing a different sequence. The switch from normal sequence to disrupted knockout sequence is just one example of a conditional allele.
The Generation of a Conditional Allele in Mice
Precise and specific genetic modifications must be made in order to create conditional alleles and the process is too complex to describe in brief. To focus on the important details, a specific DNA sequence called loxP is the crucial element of what is a conditional allele. For example, a conditional knockout allele requires the placement of two copies of the loxP sequence in a target gene. Other conditional allele designs use the loxP sequence in different ways. The methods for creating conditional alleles in mice are constantly being improved. It’s possible that new developments, such as the CRISPR/Cas9 method, may make it easier to create conditional alleles in the future.