The largest family is by far the Rhodopsin family, one reason being that it includes the many olfactory receptors (ORs).
This type of gene is very difficult to find at the receptor level because of the combinatorial coding of olfactory information, which utilizes many olfactory receptors with differential specificities to capture a single odor.
For example, dogs use smell for hunting, and, consequently, they have many more olfactory receptors than humans do and are thousands of times more sensitive to various odours.
An adult male cockroach can have as many as 195,000 olfactory receptor cells on one antenna, and an adult male tobacco hornworm moth may have from 100,000 to more than 300,000 receptor cells on one antenna.
This limited genomic material translates into a contraction of gene number, shrinking known to affect many gene families, including olfactory receptor gene repertoires.
Not only are there many different types of olfactory receptor involved in processing a complex olfactory scene [25], but absolute olfactory stimulus concentration also appears to contribute much more to stimulus identity than do absolute stimulus levels (e.g. luminance) in other sensory modalities [11], [16] [18].
Other suggestive loci were observed on chromosome 6q22-23 near LAMA2 and ARHGAP18 genes, chromosome 8q24 near the HAS2 and HAS2AS genes, and chromosome 11p15 in a LD block containing many genes, including several olfactory receptor genes.
Many gene families, including olfactory receptor, solute carrier, cytochrome P450, MHC and interleukin, which had been reported to be influenced by CNVs in human and other mammals [ 10, 44, 46], were also found to be in the CNVRs of this study.
But in Old World monkeys and in the great apes, gorillas, chimpanzees, and humans, as many as 70 percent of the olfactory receptor genes, though still identifiable, are nonfunctional pseudogenes.
