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Alleles and conditional alleles
Before we go deeper into 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. Simply put, an allele is a variant of a gene. The word allele could refer to the different gene sequences that occur naturally in a population, or in an animal model it can be a change that has been deliberately introduced. Normally, the sequence of a gene is the same in almost every cell of the body. In the early days of genetically modified mice, this also applied to genetic changes that researchers introduced. Certain research questions, however, require an animal model in which a gene has a different allele in different cells. For example, it may be necessary for most cells to have a functional copy of a gene, while only a few cells contain a non-functional copy. Conditional alleles are therefore designed to allow control over exactly when and where different alleles will be expressed.
To compare a knockout allele and a conditional knockout allele
The simplest way to explain what a conditional allele is is by directly comparing two related mouse model types, knockouts and conditional knockouts. Knock-out mice are a type of mouse model in which a specific gene has been modified to permanently disrupt its function. This disruption or knock-out affects the gene in every cell of the body of the mouse and at all stages of its life. A common way to disable a gene is to remove part of the sequence - this is a permanent change that can not be undone. Such models have proved very useful in many studies, but their limitations may stand in the way of certain experiments. A conditional knockout allele can achieve the same result, a disruption of the function of the gene, but with more control. This is because the conditional knockout allele will initially function just like the natural unmodified gene. A part of the gene can be removed just like the knock-out allele, but the removal must be activated by researchers. This principle applies to all conditional alleles: they initially represent a sequence and can then be switched to express a different order. The switch from normal sequence to disrupted knockout sequence is only an example of a conditional allele.
Generating a conditional allele in mice
Accurate and specific genetic modifications must be made to create conditional alleles and the process is too complicated to describe briefly. 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. Themethods for making conditional alleles in mice are constantly being improved. It is possible that new developments, such as the CRISPR / Cas9 method, make it easier to make conditional alleles in the future.
