The concept of "diluting" hybrids by crossing them back to either parent also developed in the latter part of the nineteenth century. This strategy was introduced to ameliorate undesirable characters that were expressed too strongly. Luther Burbank, based in California, became a master of this art. He bred larger walnuts from hybrids involving Juglans californica, J. regia, and J. nigra. From the 1870s onwards, he was especially successful with plums bred by hybridization of native American plums with a Japanese species, (Prunus triflora). Burbank once found a Californian poppy (Eschscholtzia californica) that displayed a crimson thread through one petal. By repeated selection he eventually developed an all-crimson poppy. His series of hybrids between blackberry and raspberry also produced some remarkable plants. The Primus blackberry (from western dewberry and Siberian raspberry) produced larger fruit that ripened many weeks in advance of either parent, while out-yielding both and maintaining flavor. By the turn of the century, Burbank was justly famous for having bred numerous superior cultivars of many different kinds of plants of horticultural and agricultural importance.
In genetic terms, there are two kinds of back-crossing. When one parent of a hybrid has many recessive characters, these are masked in the F1 (first filial) hybrid generation by dominant alleles from the other parent. However, a cross of the F1 hybrid with the recessive parent will allow the complete range of genetic variation to be expressed in the F2 progeny. This is termed a test cross. A cross of the F1 to the parent with more dominant characters is termed a back cross.
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