Among odor receptor genes, how many are inactive?

Among odor receptor genes, how many are inactive?

• A. About 0
• B. About 350
• C. About 650
• D. About 1,000

Answer: (C)

A large portion of odor receptor genes in the human genome are inactive because they exist as pseudogenes. The odorant receptor family is huge—roughly about a thousand gene sequences—but many have acquired disabling mutations that prevent them from producing functional receptors. As a result, only a minority remain functional, while a substantial number are inactive, around 600–650. So, about 650 inactive genes is the best-fit estimate. This pattern comes from how gene duplication creates many copies and then mutations accumulate over time, turning many copies into nonfunctional pseudogenes. In other species, the balance can shift, but in humans the high count of inactive odor receptor genes reflects this pseudogenization process.