between 1600 and 1900, so even partial protection against the disease could account for the current gene frequency. The most recent hypothesis, published in the Journal of Theoretical Biology, proposed having a single CF mutation granted respiratory advantage for early Europeans migrating north into the dusty wasteland left by the Last Glacial Maximum. As of 2016, the selective pressure for the high gene prevalence of CF mutations is still uncertain, and may be due to an unbiased genetic drift rather than a selective advantage. Approximately one in 25 persons of European descent is a carrier of the disease, and one in 2500 to 3000 children born is affected by Cystic fibrosis. Triosephosphate isomerase Triosephosphate isomerase (TPI) is a central enzyme of glycolysis, the main pathway for cells to obtain energy by metabolizing sugars. In humans, certain mutations within this enzyme, which affect the dimerisation of this protein, are causal for a rare disease, triosephosphate isomerase deficiency. Other mutations, which inactivate the enzyme (= null alleles) are lethal when inherited homozygously (two defective copies of the TPI gene), but have no obvious effect in heterozygotes (one defective and one normal copy). However, the frequency of heterozygous null alleles is much higher than expected, indicating a heterozygous advantage for TPI null alleles. The reason is unknown; however, new scientific results are suggesting cells having reduced TPI activity are more resistant against oxidative stress. PlosOne, December 2006 Resistance to hepatitis C virus infection There is evidence that genetic heterozygosity in humans provides increased resistance to certain viral infections. A significantly lower proportion of HLA-DRB1 heterozygosity exists among HCV-infected cases than uninfected cases. The differences were more pronounced with alleles represented as functional supertypes (P = 1.05 × 10−6) than those represented as low-resolution genotypes (P = 1.99 × 10−3). These findings constitute evidence that heterozygosity provides an advantage among carriers of different supertype HLA-DRB1 alleles against HCV infection progression to end-stage liver disease in a large-scale, long-term study population. MHC heterozygosity and human scent preferences Multiple studies have shown, in double-blind experiments, females prefer the scent of males who are heterozygous at all three MHC loci. The reasons proposed for these findings are speculative; however, it has been argued that heterozygosity at MHC loci results in more alleles to fight against a wider variety of diseases, possibly increasing survival rates against a wider range of infectious diseases. The latter claim has been tested