Studies of the frequency of heterozygous carriers for common inherited
diseases of haemoglobin in over 7500 adolescent children in 25 districts in
Sri Lanka have disclosed a highly significant variation over very short geo-
graphical distances. A further analysis of these findings, including their
relationship to the past frequency and distribution of malaria, climatic vari-
ation, altitude, ethnic origin and consanguinity rates, have provided evi-
dence regarding the evolutionary basis for the variable distribution of these
conditions over short distances. It is likely that the complex interplay
between malaria and the environment, together with related ethnic and
social issues, exists in many countries across the tropical belt. Hence, these
observations emphasise the importance of micromapping heterozygote dis-
tributions in high-frequency countries in order to define their true burden
and the facilities required for the prevention and management of the
homozygous and compound heterozygous disorders that result from their
interaction.