Introduction
About 1% of native people in East Asia, 5% of those in Africa, and 16% of those in Europe have an Rh-negative blood type – they carry two copies of the mutated allele of the RHD gene and therefore lack the most dominant D epitope on the surface of their erythrocytes (Flegel, 2011; Wagner & Flegel, 2000). Rh-positive subjects carry either one or two unmutated alleles of the gene. The main role of Rh complex is to transport NH3 or CO2 gases and their ions (Kustu & Inwood, 2006; Nakhoul & Hamm, 2013) but the physiological role of this transport remains unknown. For a long time, the stable coexistence of two alleles of the gene has been an evolutionary riddle. Before the discovery of modern prophylaxis, carriers of the rarer phenotype, either Rh-negative women in a predominantly Rh-positive population or Rh-positive men in a predominantly Rh-negative population, expressed lower fecundity because part of their latter-born babies (Rh-positive babies born to Rh-negative mothers) died of hemolytic disease of newborns (Fisher, Race, & Taylor, 1944; Haldane, 1942).
During the past 15 years, several methods demonstrated that RHD polymorphism is sustained in the population by balancing selection, namely by the selection in favor of heterozygotes. One group of studies showed that Rh-positive heterozygotes have better reaction times than Rh-positive and negative homozygotes when latently infected by the protozoan parasite Toxoplasma gondii (Flegr, Novotná, Lindová, & Havlíček, 2008; Novotná et al., 2008) . Currently, only about one-third of the population is infected with this parasite and the prevalence is decreasing at the rate of about 1% per year in most developed countries (Pappas, Roussos, & Falagas, 2009; Tenter, Heckeroth, & Weiss, 2000). However, it is highly probable that the majority of people were infected in our recent evolutionary past. Later, an ecological study showed that frequency of heterozygotes negatively correlates with the incidence of many diseases and with morbidity of other diseases assessed based on DALY (Disability Adjusted Life Years). The study controlled for five potential confounding variables: GDP, latitude (distance from the equator), humidity, medical care expenditure per capita, and frequency of smokers (Flegr, 2016). Other evidence of the effects of Rh phenotype on human health were obtained in several cross-sectional studies. The most detailed study performed on 3,130 subjects showed that Rh-negative subjects scored significantly worse in 6 of 22 ordinal health-related variables than Rh-positive subjects (Flegr, Hoffmann, & Dammann, 2015). The results also showed that Rh-negativity was positively associated with the incidence of 21, and negatively with the incidence of 10 of 154 diseases under study. However, most cross-sectional studies compared the health and performance of subjects with Rh-positive and Rh-negative phenotypes and all relied on the information provided by participants of anonymous internet studies. The only exception is a recent study performed on 2539 subjects (Flegr, Toman, Hula, & Kankova, 2020). In this study, respondents were asked not only about their Rh phenotype but also Rh phenotype of their biological parents. This enabled the identification of a substantial part of heterozygotes – Rh-positive subjects with an Rh-negative mother or father. The study showed that Rh-positive heterozygotes have better health than Rh-negative homozygotes and also brought very strong evidence that they have better health even in comparison with Rh-positive homozygotes.
The main purpose of the present study is to confirm the effect of Rh-genotype on physical and mental health using a non-anonymous study with subjects whose Rh-genotype was determined by the direct molecular method in a laboratory. For this purpose, we analyzed the physical and mental health-related data of 178 women and 86 men, the representatives of the nonclinical Prague population, who consented to participate in the study performed at the Faculty of Science, Charles University.