Flower colour is controlled by a single pair of alleles. The allele for red flowers is dominant to the allele for white flowers. A plant homozygous for red flowers is crossed with a plant homozygous for white flowers. All the resulting plants have red flowers (F₁ generation). When the F₁ generation are crossed with each other, 18 plants are obtained. 12 plants have red flowers and 6 have white flowers (F2 generation). What ratio is expected in the F2 generation and what ratio has been obtained?

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