The Punnett Square
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A Punnett square is a grid crossing one parent's possible gametes against the other's, showing every offspring genotype and its expected frequency.
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A 2-by-2 Punnett square for a Yy-by-Yy cross shows one YY, two Yy, and one yy square, a 1:2:1 genotype ratio.
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A Punnett square shows the population-level expected ratio — it does not predict any one individual offspring's actual genotype.