In the past, the overwhelming majority of sheep in Australia were wool Merinos. For many farmers, 'sheep' meant Merinos, and the all others were called "crossies", even if they were from a pure breed with a long history. Most sheep in Australia are still Merinos, but meat lambs are now more profitable than wool for many farmers. Sires from meat breeds have improved the meat lambs and the profits. Their lambs are 'cross-breds', and the ewes may be first or second cross.

Virtually every sheep breed has been developed through cross-breeding. A few old breeds have been 'pure' for a very long time, and almost all of them are horned. Horned sheep have some fans, but most farmers prefer polled stock. Merino flocks always had a few sheep which had a recessive poll gene. From hundreds of thousands of Merinos, farmers found enough with this gene to breed Poll Merino flocks.

Throughout the 20th century 'breeding out the horn' (or 'breeding to poll') continued in both sheep and cattle. As a result, very few modern breeds are really 'pure'. One after another, they were bred to poll by cross breeding to another polled breed. The aim was normally to keep all the genes of the original breed except the horn gene.