Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 289

Genetically divergent parents are crossed for recombination of their desirable genes. To optimize results, parents should be carefully selected to have a maximum number of positive traits and a minimum number of negative traits with no negative traits in common (i.e. elite × elite cross). This way, recombinants that possess both sets of desirable traits will occur in significant numbers in the F₂. The F₁ contains the maximum number of desirable genes from both parents. There are several ways to conduct divergent crosses (Figure 6.2):

 Single crossIf two elite lines are available that together possess all desired traits at adequate levels, one cross (single cross [A × B]) may be all that is needed in the breeding program.

 Three‐way crossSometimes, for combining all desirable traits several cultivars or elite germplasm are required, since each pair may have some negative traits in common. In this case, multiple crosses may be required in order to have the opportunity of obtaining recombinants that combine all the desirable traits. The method of 3‐way crosses ([A × B] × C) may be used. If a 3‐way cross product will be the cultivar, it is important that especially the third parent (C) be adapted to the region of intended use, since it contributes more genes than each of the A and B parents.

 Double crossA double cross is a cross of two single crosses ([A × B] × [C × D]). The method of successive crosses is time consuming. Further, the complex crosses such as double cross have a low frequency of yielding recombinants in the F2 that possess a significant number of desirable parental genes. When this method is selected, the number of targeted desirable traits should be small (at most about 10). The double‐cross hybrid is genetically more broad‐based than the single‐cross hybrid but is more time consuming to make.

 Diallel crossA diallel cross is one in which each parent is crossed with every other parent in the set (complete diallel), yielding n − (n−1)/2 different combinations (where n = number of entries). This method entails making a large number of crosses. Sometimes, the partial diallel is used in which only certain parent combinations are made. The method is tedious to apply to self‐pollinated species. Generally, it is a crossing method for genetic studies, and less for the purpose of creating populations for breeding.

Figure 6.2 The basic types of crosses used by plant breeders. Some crosses are divergent (a) while others are convergent (b).