Congenic Mouse Strains Explained

The question asks about two strains of mice that are genetically identical except for a single genetic locus or region. Understanding different types of mouse strains used in research is key to answering this.

Let's look at the options provided:

• Syngenic: This term refers to individuals or strains that are genetically identical. For example, individuals from a highly inbred strain or identical twins are syngenic. While the strains start genetically identical in the process described in the question, the end state is not purely syngenic because there is a difference at a single locus.
• Allogenic: This term refers to individuals or tissues that are genetically distinct within the same species. This is the opposite of what the question describes.
• Congenic: This term specifically describes strains of animals (like mice) that are genetically identical to another strain (the background strain) except for a single genetic locus or a small chromosomal region that has been introduced from a different strain. These are typically created through a process of repeated backcrossing. This perfectly matches the description in the question.
• Heterogenic: This term generally refers to having diverse genetic origins or composition, implying genetic variation rather than near genetic identity with a difference at a single locus.

Based on the definitions, the term that accurately describes two strains of mice that are genetically identical except for a single genetic locus or region is Congenic.

Creating Congenic Strains

Congenic strains are usually developed by repeatedly mating a donor strain (carrying the desired gene/locus) with a recipient inbred strain (the background strain). Offspring carrying the desired gene from the donor are selected in each generation and backcrossed to the recipient strain. After many generations (typically 10 or more), the resulting strain is genetically identical to the recipient background strain everywhere except for the small region containing the desired gene from the donor. This process isolates the effect of a specific gene or locus on a consistent genetic background.

Therefore, two strains of mice that are genetically identical except for a single genetic locus or region are called Congenic.