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When a marker is genetically linked to a trait, its use can speed up the identification of genetically superior plants.

DNA can be extracted from very young plants and the marker assay carried out long before the  plant expresses the actual trait.

DNA can be extracted from plants at a very early stage, much sooner than most traits can be seen or measured.  This person is harvesting leaves for DNA extraction from tomato seedlings just a few days after germination.