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Mendelian genetics, Punnett square. Diagram showing the genetics of seed colour and shape in peas, as discovered by Gregor Mendel in the 1850s and 1860s. The colour genes are Yy (yellow-green) and the shape genes are Rr (round-wrinkled). Yellow (Y) and round (R) are dominant. The pure parent lines (top) combine gametes (pollen and eggs) to produce a first generation (F1) of yellow round seeds (YyRr). This self-fertilises to produce the second generation (F2). The colour and shape ratios in this F2 generation are shown in the form of a Punnett square, with the ratios being 9:3:3:1 of phenotypes (yellow round, yellow wrinkled, green round, and green wrinkled).
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