How many generations of brother-to-sister matings are needed to establish an inbred strain?

• A. A minimum of 5
• B. A minimum of 10
• C. A minimum of 20
• D. A minimum of 30

Answer: (C)

Repeated full-sib mating gradually increases homozygosity across the genome. An inbred strain is defined by nearly complete homozygosity, meaning almost all gene pairs are identical by descent. It takes about twenty generations of brother-to-sister matings to reach that level, roughly achieving an inbreeding coefficient near 0.99. Before twenty generations, enough heterozygosity remains that the line is not yet considered inbred, and more generations beyond twenty are not typically necessary for practical uses. That’s why twenty generations is the minimum to establish an inbred strain.