when a population is exposed to the same infectious agent, not all individuals show susceptibility to the infection. To find out the reason, M J Stear and colleagues of the Glasgow University Veterinary School, uk , investigated the basis of resistance to nematode infection in sheep. Nematode worms are major parasites of plants and animals that cause lethal infections ( Nature , Vol 389, No 6646).

The researchers monitored sheeps from the same flock of a commercial farm. They studied them for the load of nematode worms, which the sheeps carried during their life span and also at the time of death. Most of the parasites belonged to a single specie called Ostertogia circumcincta . At least 25 O circumcincta were measured from each sheep and data were analysed to find whether sheeps that were genetically related to each other at a level higher than the average degree of relatedness in the flock, also resembled each other in respect to some property of the parasites that they carried.

The researchers also estimated heritability