The specific high intelligence pattern seen in the genetically distinct population of Ashkenazi Jews is attributed to a heterozygote advantage of genetic diseases that frequent this population. Mutations that cause increased fitness for a heterozygote while causing a great decrease in fitness for a homozygote occur as a response to a unique environment (Hammerstein 1996). A classic example of this is sickle cell anemia that involves hemoglobin and malaria resistance in the Old World tropics. It has been proposed that clusters of Ashkenazi genetic diseases, the DNA repair cluster and the sphingolipid cluster, cause an increase in intelligence of heterozygotes (Cochran et al. 2006). Heterozygotes for these mutations show increased levels of storage in brain cells, increased brain cell growth, and have significantly higher intelligences levels than other European populations (Cochran et al. 2006). It is believed that the genetic diseases are a by-product of selection for higher intelligence. It has been proposed that these clusters of genetic diseases arouse due to strong and recent selection pressures, as the disease clusters affect specific pathways and contain many alleles (Cochran et al. 2006). This shows that they were not produced by a bottleneck followed with genetic drift as previously believed. The selection experienced is that of a unique social niche during 18th century Europe, which selected for high cognitive ability (Ostrer 2001). This theory of selection for intelligence in Ashkenazi Jews is a great modern day example of the heterozygote advantage in a complex trait.
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