Over the last twenty years the British dairy population has undergone a large importation of Holstein genes, largely from North America and Canada. This study aimed to investigate what effect this importation has had on the genetic diversity of the population and also to look at the change in genetic diversity of the population over the last thirty years.

As computing resources did not facilitate sampling of all cows born in the last thirty years, between 4 and 10 random samples of 2000 Holstein-Friesian cows, born every 5 years from 1967 to 1997 inclusive, were taken from the Holstein Friesian Society of Great Britain and Ireland's database. The base population of this database was born about 1960. The complete ancestry of these cows was traced on the database and these sample pedigrees were used for the various analyses.