What percentage of the genes in the genome of a mouse have a corresponding gene in the genome of humans?

• A. 20%
• B. 40%
• C. 80%
• D. 100%

Answer: (C)

Genes are often conserved across mammals because we share a common ancestor, so many mouse genes have human counterparts that perform similar roles. This is the idea of orthologs—genes in different species that originated from the same ancestral gene and retain related functions. Because of this evolutionary connection, roughly four out of five mouse genes have a recognizable corresponding gene in humans.

The reason it isn’t closer to 100% is that some genes are unique to mice or have diverged so much that a clear human counterpart isn’t identifiable, and others have duplicated or been lost in one lineage over time. This conservation underpins, but also limits, how perfectly mouse biology models human biology.