the amygdala and occipital cortex is triggered. This chain reaction serves to help assess the nature of the threat and increase chance of survival. Humans have few olfactory receptor cells compared to dogs and few functional olfactory receptor genes compared to rats. This is in part due to a reduction of the size of the snout in order to achieve depth perception as well as other changes related to bipedalism. However, it has been argued that humans may have larger brain areas associated with olfactory perception compared to other species. Genes affecting body odor MHC Body odor is largely influenced by major histocompatibility complex (MHC) molecules. These are genetically determined and play an important role in immunity of the organism. The vomeronasal organ contains cells sensitive to MHC molecules in a genotype-specific way. Experiments on animals and volunteers have shown that potential sexual partners tend to be perceived more attractive if their MHC composition is substantially different. Married couples are more different regarding MHC genes than would be expected by chance. This behavior pattern promotes variability of the immune system of individuals in the population, thus making the population more robust against new diseases. Another reason may be to prevent inbreeding. ABCC11 The ABCC11 gene determines axillary body odor and the type of earwax. The loss of a functional ABCC11 gene is caused by a 538G>A single-nucleotide polymorphism, resulting in a loss of body odor in people who are specifically homozygous for it. Firstly, it affects apocrine sweat glands by reducing secretion of odorous molecules and its precursors. The lack of ABCC11 function results in a decrease of the odorant compounds 3M2H, HMHA, and 3M3SH via a strongly reduced secretion of the precursor amino-acid conjugates 3M2H–Gln, HMHA–Gln, and Cys–Gly–(S) 3M3SH; and a decrease of the odoriferous steroids androstenone and androstenol, possibly due to the reduced levels and secretion of DHEAS and DHEA (possibly bacterial substrates for odoriferous steroids). Secondly, it is also associated with a strongly reduced/atrophic size of apocrine sweat glands and a decreased protein (such as ASOB2) concentration in axillary sweat. The non-functional ABCC11 allele is predominant among East Asians (80–95%), but very low in other ancestral groups (0–3%). Most of the world's population has the gene that codes for the wet-type earwax and average body odor; however, East Asians are more likely to inherit the allele associated with the dry-type earwax and a reduction in body odor.