Maternal effect: an additional mechanism maintaining balanced polymorphisms of haemoglobinopathies?

The malarial hypothesis presupposes that carriers of sickle cell trait and beta-thalassaemia are born in accordance with Mendelian inheritance and later favoured by an environmental factor. However, an eventual distortion favouring the transmission of the mutant allele is also a plausible hypothesis. The Mendelian proportion was tested in the progeny of 201 sickle cell trait (AS) and 138 beta-thalassaemia (AT) probands married to individuals with normal haemoglobin (AA). An excess of births of heterozygotes in the offspring of the 107 AS mothers (144 AS:89 AA, chi2=12.98; p<0.001) and 95 AT mothers (117 AT:66 AA, chi2=14.21; p<0.001) married to homozygous normal fathers was observed, but not for births to the reverse parental combinations (103 AS: 101 AA, chi2=0.019; p=0.89 and 57 AT:42 AA, chi2=2.27; p=0.13). Such data reveal a statistically significant maternal effect favouring the transmission of haemoglobin S and beta-thalassaemia alleles. Obviously, prezygotic and postzygotic mechanisms of distortion cannot be ignored and need further investigation. However, the selective embryonic survival of AS and AT conceptuses of heterozygotic mothers, with an excess of unrecognized very early embryonic deaths among the AA conceptuses is a very attractive hypothesis that should also be taken into consideration.