The answer to this question is not straightforward. It depends on several factors, including the genetic background of the populations, the heritability of the trait, and the stability of the QTL across different environments. In general, it is possible to use a QTL identified in one population for MAS selection in another population, but caution must be taken to ensure that the QTL is transferable and has a significant effect on disease resistance in the new population. This can be done by conducting validation experiments using a large number of individuals from both populations and testing for association between the QTL and disease resistance. If the QTL is found to be stable and transferable, it can be used as a marker for MAS selection in the new population. However, if there are significant differences in genetic background or environmental conditions between the two populations, it may be necessary to identify new QTLs specific to the new population for effective MAS selection. Ultimately, careful consideration of these factors is necessary when using QTLs.

Keep in mind that a QTL is detected specifically when the offspring are segregating for different alleles of a gene affecting the trait. E.g. if parents A and B are fixed for functionally different alleles, their F2 offspring will segregate for AA, AB and BB combinations of alleles at a QTL with different degrees of disease resistance. When you cross parents C and D, of both parents are homozygous for a resistance allele or if both are lacking a resistance allele, there will be no QTL effect.

Dear Dr. David L Remington you answered excelent. However, if one parent has a resistance allele and the other does not, the offspring will segregate for resistance and susceptibility alleles at the QTL. This allows for the detection of QTLs that affect disease resistance in the offspring. QTL mapping is a powerful tool in plant breeding, as it allows breeders to identify regions of the genome that are associated with desirable traits such as disease resistance. By identifying these regions, breeders can use marker-assisted selection to select for plants with desirable traits more efficiently. Additionally, QTL mapping can also provide insights into the genetic basis of complex traits and help researchers understand how different genes interact to affect trait expression. Overall, QTL mapping is an important technique in plant breeding and genetics research that has numerous applications in improving crop yields and quality.

In general, markers of QTL detected in the population of parents A and B can be used to detect another population (when parents C and D have polymorphism on this QTL (marker)).

Another consideration (when I read your question more carefully): If you're simply doing MAS using the same genetic marker in another cross (CxD) without actually scoring the progeny phenotypes in the new cross, there's no expectation that the same marker alleles are associated with the same QTL alleles in the new cross. Unless C and D are extremely closely related to A and B respectively, many generations of crossing over will have occurred and probably changed which marker alleles are linked to which QTL alleles. Unless you're really lucky or actually know the gene that is responsible, the genetic marker is only linked to the actual QTL, not in it.