True-Breeding Lines and Controlled Pollination
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A true-breeding line always produces offspring with the parent's trait when self-pollinated, and Mendel controlled pollination by hand to fix each cross.
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Mendel removed a flower's anthers before they matured, then applied pollen from a chosen second plant.
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Assuming any parent plant showing a trait is true-breeding for it is the mistake — a hidden heterozygote scrambles the cross.