25. Two strains of mice which are genetically identical except for a Single genetic locus or region are said to be:
(1) Syngenic       (2) Allogenic
(3) Congenic      (4) Heterogenic

Concept

When geneticists want to study the effect of a single gene without background noise from other loci, they use pairs of strains that are virtually identical everywhere in the genome except at one locus (or a very small linked region). These are called congenic strains. They are produced by repeated backcrossing of a “donor” allele into a defined inbred “recipient” background until all other donor DNA is eliminated.

Such congenic pairs allow clean comparisons of phenotypes that can be attributed to that one genetic difference.

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Option‑wise explanation

1. Syngenic

   • Often used loosely to mean “genetically compatible” or very similar, but in classical mouse genetics the precise term for strains differing only at a defined locus is congenic, not syngenic.

2. Allogenic

   • Means “genetically different” or “from another individual/strain” in transplantation or immunology contexts (e.g., allogeneic grafts).

   • It does not imply that the strains are identical except for one locus.

3. Congenic – correct

   • Defined as strains that are genetically identical except for one locus or a small chromosomal segment introduced from another strain via backcrossing.

   • This matches exactly the description: “genetically identical except for a single genetic locus or region.”

4. Heterogenic

   • Refers broadly to being genetically heterogeneous or mixed, the opposite of what is described.

   • It does not denote a carefully controlled single-locus difference.

Therefore, two mouse strains that are identical except for one locus or small region are called congenic strains (option 3).