Frequency in US from Chinevere et al⁴⁴ ; in Nigeria, from Luzzatto and Allan⁴⁵ ; in Sardinia, from Cocco et al⁴⁶ ; in Thailand, from Bancone et al.⁴⁷  The frequency of G6PD deficiency in hemizygous males is identical to the frequency of the G6PD mutant allele (or the sum total of the frequencies if more than 1 mutant allele is involved). From the male frequency, it is easy to calculate the frequencies of the female genotypes, on the assumption that the population is in Hardy-Weinberg equilibrium. The data in this table show that, as all textbooks say, homozygous females are few compared to hemizygous males; on the other hand, heterozygous females are almost twice as common as hemizygous males (which most textbooks fail to say). Because of epigenetic mosaicism consequent on X chromosome inactivation, all heterozygotes have some G6PD deficient red blood cells: on average 50%, but with a wide range of variation. This is why in heterozygotes clinical manifestations are, on average, less severe, but not always so. In every series of patients with favism, there are always heterozygous girls.¹⁹