Genetic issues

With each backcross, the progeny becomes more like the recurrent parent. In theory, the BC4 genotype will be 93.75% identical to the recurrent parent. The mathematical relationship for the recovery of the recurrent parent is presented by W. Allard is: 1 _ 1=2 _ _m_1 where m is the number of generations of selfing or backcrosses. In another way, the proportion of the donor genes is reduced by 50% following each generation of backcrossing. This is obtained by the relationship m 1, where m is the number of crosses and backcross to the parent. For example, in the BC4, the value is 5ผ3.125%. To obtain the percentage of homozygotes for alleles of recurrent parents in any generation, the mathematical relationship is: ฝ 2m_1=2m_n where n is the number of genes. Because of cytoplasmic inheritance, it is sometimes critical which of the two parents is used as female. For example, to use CMS in breeding, the male fertile inbred lines with normal cytoplasm and non-restorer genes are converted to sterile cytoplasm to be used as male sterile female lines in a cross. The resulting cultivar from a backcross breeding program could differ from the starting cultivar beyond the transferred genebecause of linkage drag from the association of undesirable traits with the genes from the donor. Backcrossing is more effective in breaking linkages over selfing, especially where heritability is low for the undesirable trait.

A certain number of individuals is needed for a chance to recover the desired genes in a backcross program. This number increases as the number of genes controlling the donor trait increases. Furthermore, for multiple gene traits, it will be necessary to grow backcross progeny through F2 or later generations to obtain the desired genotypes for advancing the program. When the trait is governed by a dominant gene, it is easy to identify plants carrying the desired gene. However, when the desired trait is conditioned by a recessive gene, an additional step is needed after each backcross to produce an F2 generation in order to identify the recessive trait. The genetic advance in backcross breeding depends on several factors:

_ Heritability of the trait. As previously indicated,traits that are conditioned by major genes and have high heritability are easier to transfer by backcross.

_ Sustainable intensity of trait expression. Progress with selection will be steadier where the expression of the trait of interest remains at a high intensity

throughout the program.

_ Availability of selection aids. Ability to identify and select desirable genotypes after the backcross is critical to the success of the procedure. Depending

on the trait, special selection techniques may be needed. For disease resistance breeding, artificial disease epiphytotic may be necessary. Molecular markers may be helpful in selection reducing the number of backcrosses needed for the program.

_ Number of backcrosses of marker. The genetic distance between the parents is important to the progress made in backcrossing. If both are closely related cultivars, fewer backcrosses would be needed than if the gene transfer is from a wild genotype to an adapted one.