All you users of the scent test are going to love what Siddhartha Mukherjee writes in his amazing book, THE GENE. The only parts I have inserted are between square brackets [..] for clarity to identify the noun associated with the pronoun. The rest are all his words.
p. 323 “Parts of it [the genome] are surprisingly beautiful. On a vast stretch on chromosome eleven, for instance, there is a causeway dedicated entirely to the sensation of smell. Here, a cluster of 155 closely related genes encodes a series of protein receptors that are professional smell sensors. Each receptor binds to a unique chemical structure, like a key to a lock, and generates a distinctive sensation of smell in the brain — spearmint, lemon, caraway, jasmine, vanilla, ginger, pepper. An elaborate form of gene regulation ensures that only one door-receptor gene is chosen from this cluster and expressed in a single smell-sensing neuron in the nose, thereby enabling us to discriminate thousands of smells.”
p.325 “Its [The genome’s] first gene, on chromosome one, encodes a protein that senses smelling the nose (again: those ubiquitous olfactory genes!). Its last gene, on chromosome X, encodes a protein that modulates the interaction between cells of the immune system. (The “first” and “last”) chromosomes are arbitrarily assigned. The first chromosomes is labeled first because it is the longest.”