Twins, for example, will be smaller and grow less wool than singles. Earlier born lambs will cut more wool than later lambs and the lambs of hogget ewes will be disadvantaged compared with lambs reared by older ewes. This means that a twin lamb born late in the drop to a hogget mother will have to be very good genetically to perform at the same level as a single lamb born early to a mature ewe. Each animal is therefore the product of its genetic value combined with a certain amount of luck. Bad luck can make an animal with good genes look average or a genetically average animal look poor.
Good luck can make an animal with average genes look a better breeding prospect than it really is.
This combination of genes and luck is what controls heritability. In a highly heritable trait, luck and genes are of nearly equal value in determining an animal's ranking within the flock. The lower the heritability of the trait, the greater the importance of luck, and the harder it becomes to predict progeny performance accurately. The most important decision faced by breeders and their advisers is which animals should be kept as parents for the next generation. Rapid genetic progress is only possible if the very best rams are selected as reserve sires.