About how many odor receptors are active within the gene family?

• A. 350
• B. 100
• C. 520
• D. 210

Answer: (A)

Odor receptor genes form a very large gene family, but not all of them are functional. In humans, only a subset of these genes are actively expressed as functional receptors in the nose, with estimates commonly landing around a few hundred. The figure closest to this practical, real-world count is about 350 receptors, which is why that option best matches what scientists typically report. The rest of the family consists of nonfunctional copies (pseudogenes) or genes not expressed in the olfactory system, so the total family size is larger than the number of active receptors. The other numbers are farther from the usual functional-receptor estimate, making 350 the most reasonable choice.