55. Marker-assisted selection (MAS) defined as selection based on molecular markers should have some important criterion for plant breeding activities. Some statements about these criteria are mentioned below:
A. Marker should co-segregate with the desired trait of interest.
B. Marker should not co-segregate with the desired trait of interest.
C. Marker should be un-linked with the desired trait of interest.
D. Marker is used for indirect selection of a genetic determinant or determinants of a trait of interest.
Which one of above combinations is correct?
(1) A and B         (2) B and C
(3) C and D        (4) A and D

Marker-assisted selection (MAS) is a plant breeding method that selects plants based on molecular markers linked to desired traits. The correct criteria for MAS are that the marker should co-segregate with the desired trait (meaning it physically links with the trait gene during inheritance) and that the marker is used for indirect selection of the genetic determinants of that trait. Hence, statements A (“Marker should co-segregate with the desired trait of interest”) and D (“Marker is used for indirect selection of a genetic determinant or determinants of a trait of interest”) are correct.

Explanation of the options:

• A. Marker should co-segregate with the desired trait of interest: Correct because MAS relies on markers tightly linked to the trait gene so that selection on the marker reflects the selection for the trait.

• B. Marker should not co-segregate with the desired trait of interest: Incorrect because if a marker is not co-segregating (not linked), it will not reliably indicate the presence of the desired trait.

• C. Marker should be un-linked with the desired trait of interest: Incorrect because an unlinked marker does not predict the trait inheritance, making MAS ineffective.

• D. Marker is used for indirect selection of a genetic determinant or determinants of a trait of interest: Correct and fundamental to MAS, as markers serve as proxies for the genetic region controlling the trait.

The correct combination is therefore (4) A and D.

Introduction:
Marker-assisted selection (MAS) is revolutionizing plant breeding by enabling indirect selection of desirable traits through molecular markers. This approach depends critically on specific criteria to ensure markers accurately predict the inheritance of important traits, enhancing the efficiency and precision of breeding programs.

Detailed Explanation:

• Marker should co-segregate with the desired trait
  For MAS to be effective, molecular markers must be genetically linked to the trait locus, meaning they co-segregate during meiosis. This close physical linkage ensures that selection for the marker also selects plants carrying the desired trait, eliminating individuals without it early in breeding programs.

• Marker should not be un-linked and must serve indirect selection
  Markers that are un-linked or not associated with the trait provide no predictive power in selection. MAS uses these markers as an indirect selection tool, targeting genetic determinants of traits rather than the trait phenotype directly. This is especially useful for traits that are difficult to measure phenotypically or expressed late in development.

• Incorrect concepts and implications
  Selecting markers not linked or that do not co-segregate with the trait will result in poor correlation between marker and phenotype, compromising selection efficacy and genetic gains in breeding.

Conclusion:
In summary, the critical criteria for marker-assisted selection in plant breeding include the co-segregation of markers with the trait and the use of these markers for indirect selection of genetic determinants. This ensures high precision and efficiency in breeding programs aimed at trait improvement.

This detailed explanation and SEO-optimized content addresses the question comprehensively, making clear why option (4) A and D is the correct answer.