4.Polygenic traits in crops can be identified by
(1) QTL mapping
(2) Cluster analysis
(3) Tandem array analysis
(4) Gene mapping

Polygenic traits in crops are best identified by QTL (Quantitative Trait Locus) mapping. QTL mapping is a genetic analysis approach that identifies regions on chromosomes linked to quantitative traits which are controlled by multiple genes. It uses genetic linkage maps combined with phenotypic data to locate individual genetic factors influencing polygenic traits.

Explanation of Options:

1. QTL mapping: This is the primary method to identify polygenic traits. It detects quantitative trait loci that contribute to variation in complex traits controlled by multiple genes. It relies on molecular markers and linkage maps to find regions of the genome associated with the phenotype. QTL mapping is widely used in crop genetics for traits like yield, drought tolerance, and quality.​

2. Cluster analysis: This is a statistical technique used to group similar objects based on their characteristics. While it can be used to analyze phenotypic data or gene expression profiles, it does not directly identify polygenic traits or locate associated genes on chromosomes. Cluster analysis helps in data interpretation rather than gene mapping.

3. Tandem array analysis: This method relates to the study of tandemly repeated DNA sequences in the genome. It is not a standard approach for identifying polygenic traits or quantitative trait loci. It is more related to genome structure analysis than trait mapping.

4. Gene mapping: This is a broader term referring to mapping genes in general but does not specify the focus on quantitative or polygenic traits. It includes mapping single-gene traits as well. QTL mapping is a specific form of gene mapping focused on quantitative traits influenced by multiple genes.

Introduction:
Polygenic traits in crops, such as yield and stress tolerance, are controlled by multiple genes. Identifying these traits accurately is vital for crop improvement. One of the most effective techniques to discover these complex traits is Quantitative Trait Locus (QTL) mapping, which links phenotypic variation to specific regions in the genome.

Detailed Explanation:
Polygenic traits in crops can be most effectively identified through QTL mapping. This method uses genetic markers and linkage maps to locate chromosome regions associated with complex traits governed by multiple genes. Unlike cluster analysis or tandem array analysis, QTL mapping directly links genotype to phenotype for quantitative traits. General gene mapping is a broader concept but does not specifically target polygenic traits like QTL mapping. Therefore, QTL mapping is the specialized and preferred technique for detecting polygenic traits critical for breeding resilient and high-yield crops.

This comprehensive understanding helps plant geneticists and breeders develop improved crop varieties by targeting important traits controlled by many genes simultaneously.

Thus, the correct answer to the question is (1) QTL mapping.