What is the difference between inbred and outbred strains in terms of genetic variability and experimental consequences?

• A. Inbred strains are always healthier.
• B. Outbred strains are inbreeding.
• C. Inbred strains are nearly genetically identical across individuals, increasing consistency; outbred strains maximize genetic diversity, which can increase variability.
• D. Inbred strains have more genetic diversity than outbred strains.

Answer: (C)

Understanding how genetic variability shapes experimental outcomes is essential. Inbred mouse strains come from many generations of brother-sister matings, which makes them nearly genetically identical from one mouse to the next. That uniform genetic background makes observed responses more consistent within the strain, so you can attribute differences to the experimental treatment with less background noise. But extreme homozygosity can reveal or fix deleterious recessive alleles, leading to health issues and reduced ability to generalize findings to other genetic backgrounds.

Outbred strains are maintained to preserve genetic diversity; individuals differ in many genes, so responses vary more from one mouse to another. This mirrors the variation seen in real populations and can improve external relevance, but it also means experimental results have more background variability, which can require larger sample sizes to detect treatment effects and make it harder to separate treatment effects from genetic differences.

So the correct idea is that inbred strains are nearly genetically identical across individuals, increasing consistency; outbred strains maximize genetic diversity, which can increase variability. The other statements don’t fit because inbreeding reduces genetic diversity, outbred strains are not inbred, and inbred strains are not inherently healthier.