Rather than focusing on specific genes, Mackinnon and colleagues were interested in the relative contributions of host genetics and other factors to the risk of malaria. To estimate the overall contribution of genetic factors to the difference in disease incidence between individuals within a population, one needs three types of data: (1) disease incidence for individuals over a certain period of time (to be able to determine an individual's risk); (2) information on genetic relatedness of the individuals in the population; and (3) a set-up in which individuals with different levels of relatedness share the same environment and/or where related individuals live in different environments. (The third condition is essential to distinguish between genetic and environmental effects.)
The researchers studied two populations of children from a malaria-endemic area in Kenya for which they could obtain the necessary data. In one case, they determined incidence of mild clinical malaria in 640 children over a period of 5 years. For the second part, they monitored severe malaria that led to hospitalization and non-malaria hospitalizations in 2,900 children, also over a five-year period.
They found that host genetic factors accounted for about one quarter to one third of the total variation in susceptibility in the populations to malaria, and that household-related factors (i.e. environmental factors) contributed to a similar level. Overall, children living in the 10% of households with the highest malaria incidence had about twice as many infections per year than
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