Blood groups. The inheritance of the ABO blood groups in man is an instance of incomplete dominance. According to whether their blood will mix without clotting during a transfusion, people are classified into four major blood groups, A, B, AB and O. The blood group is controlled by three genes, A, B and O acting at the corresponding site on homologous chromosomes. A person will inherit two of these genes, one from each parent. Gene O is recessive to both A and B, but A and B are co-dominant, i.e. if a person inherits gene A from one parent and gene B from the other, he will be group AB, neither gene being dominant to the other. It follows that group O people must have the genotype OO while group A persons could be AA or AO and group B individuals BB or BO. The following example shows the possible blood groups of children born to a group A man and a group B woman both of whom are heterozygous for these genes.
Sickle cell anaemia occurs when two recessive mutant genes controlling haemoglobin production, combine in an individual. Such individuals have severe anaemia and generally die before reaching the reproductive age. The recessive genes remain in the population, however, in the heterozygous individuals, e.g. HH=normal; hh =sickle cell anaemia; Hh=heterozygous for sickle cell anaemia. Although from one quarter to one half of the haemoglobin of the heterozygotes is affected, usually less than l per cent of their red cells show sickling in low oxygen concentrations. Such heterozygotes are said to have the sickle cell trait. When two heterozygotes marry, one would expect on average a quarter of their children to have sickle cell anaemia and would die before reaching reproductive age.
In this way the h genes would be eliminated by selection from the population since for every four offspring from the HH genotypes there will be only three from the Hh genotypes. In certain African populations, however, as many as 34 per cent of the people carry the recessive gene and there is evidence to suggest that this is because the heterozygotes are more resistant to malaria, i.e. persons with the genotype Hh have an advantage in malarious districts while the HH genotypes are reduced by malaria. Investigations of Negro populations in America show an incidence of only 4 to 5 per cent of the trait compared with the 15 to 20 per cent characteristic of the African population from which the migrants were originally derived. In some non-malarial regions therefore, it looks as if the heterozygotes lose their advantage.
