Despite rising concern over the biology of Sickle Cell Anaemia (SCA), no general modeling approach has considered the effects of the selective advantage of Haemoglobin S (HbS) over Haemoglobin (HbA). This paper develops a theoretical framework and demonstrates mathematically that mating between the three different genotypic groups, namely: normal AA, carriers AS and sicklers SS, does not yield equal probabilities of having neonates of these classes.