11.6.6 Introgression

Males of breed B or crossed with females of breed A to incorporate a characteristic that is present in breed B with a high frequency and that is absent or has a low frequency in breed A. The females of the first cross  (F1: AB) are selected for the wanted characteristic of breed A and mated with males of breed A. This is repeated in the next generations. Breed B is used once and selection in the crossbreds leads to the introgression of the wanted characteristic of breed B. Genetic markers for the wanted characteristic improve the applicability of this method greatly. An example of introgression is the transfer of the Booroola allele in the Texel sheep breed in The Netherlands. It increases litter size and is present in Merino sheep. Crossing once Texel ewes with Merino rams followed by selection of carriers of the Booroola allele in the F1 resulted in purebred Texel ewes producing twice as many lambs compared to non-carriers of the wanted allele.

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