The numbers of offspring, when summed over years, were estimated as being sufficient to determine genetic parameters with the required precision, and provided the number of lambs that could be accommodated and raised at the farm each year. By replicating over six years a range in different weather patterns was covered.

For the purposes of this example, only the following four offspring genotypes are considered: D x D, D x R, R x D and R x R. For shorthand we shall use the abbreviation DD, DR, RD and RR, respectively, with the first letter referring to the breed of the sire and the second to the breed of the dam. Eight hundred and eighty two lambs within these genotypes were born during the six years to 74 rams and 367 ewes. Thus, each ewe gave birth on average to approximately two to three lambs, one each in a different year, before being replaced. A few twins were born but these were not included in the experiment.

Lambs were weighed and blood and faecal samples were taken periodically over a period of about a year for each of the six batches of lambs born annually during the study. Measurements were made of blood packed cell volume (PCV), which reduces in volume when an animal becomes anaemic due to disease, and faecal egg count (FEC) which estimates the numbers of helminths present in the intestines. These measurements were made monthly up to weaning at about three months of age, and on average every two to three months thereafter to about 12 months of age. The periods from birth to weaning and from weaning to 12 months are distinct periods of growth and thus required separate analysis. Further details of the experimental design are given in Baker et al. (1999 ) and Baker et al. (2003).