What best describes a congenic strain?

• A. The result of backcrossing for 10 generations
• B. The result of consecutive matings of two hemizygous transgenic parents
• C. The result of brother-sister mating of transgenic animals
• D. The result of consecutive matings of two transgenic homozygous parents

Answer: (A)

A congenic strain is created by introgressing a donor allele into a recipient strain through repeated backcrossing, so that the resulting line is nearly genetically identical to the recipient except for a defined region from the donor. After about ten backcross generations, the donor genome is diluted to only that targeted locus, leaving a background that matches the recipient strain.

This approach contrasts with the other breeding scenarios, which describe different ways of manipulating transgenes or inbreeding rather than introducing a single donor locus into a uniform background.