Of the odor receptor genes, approximately how many are active?

Of the odor receptor genes, approximately how many are active?

• A. About 1,000
• B. About 350
• C. About 50
• D. About 650

Answer: (B)

The key idea is that the odor receptor gene family in humans is enormous, but only a subset of those genes are actually functional and expressed as receptors. In the olfactory system, many gene copies have become nonfunctional pseudogenes over evolution, so the number of real, active odor receptors is much smaller than the total number of OR gene sequences. In humans, estimates place the number of functional odor receptor genes at roughly a few hundred—about 350 is a commonly cited figure. So, although the OR family may include around a thousand gene-like sequences total, only about 350 are active as receptors. The other options either overstate the active set (as if all OR sequences were functional) or understate it (too few), making the ~350 active receptors the best match for what’s actually functional.

The key idea is that the odor receptor gene family in humans is enormous, but only a subset of those genes are actually functional and expressed as receptors. In the olfactory system, many gene copies have become nonfunctional pseudogenes over evolution, so the number of real, active odor receptors is much smaller than the total number of OR gene sequences.

In humans, estimates place the number of functional odor receptor genes at roughly a few hundred—about 350 is a commonly cited figure. So, although the OR family may include around a thousand gene-like sequences total, only about 350 are active as receptors. The other options either overstate the active set (as if all OR sequences were functional) or understate it (too few), making the ~350 active receptors the best match for what’s actually functional.