Dominance

Consider two true-breeding varieties; a white pistil variety and a variety showing only pink pistils. Both conditions are true-breeding and therefore homozygous; in each case sexual reproduction of each group separately leads to only pink pistil or white pistil plants respectively. An F1 hybrid, or first generation cross of these two varieties, results in only white pistil plants; no pink pistils are seen regardless of how many Ft seeds are grown.

Upon sib mating of these Fl plants (crossing brothers to sisters, or mating Fl siblings), the resulting F2 generation produces 75% white pistil plants and 25% pink pistil plants. Notice the "disappearance" of the pink pistil plants in the Fl generation, and their subsequent "reappearance" in the F2 generation. In this case, white pistils are said to be dominant over pink pistils, and pink pistils are said to be recessive to white pistils.

Again, let's consider the phenolypes seen, and provide symbols and deduce the genotypes:

Pt - White pistils x P2 - Pink pistils i i

Fl generation (All white-pistil plants)

Fl male x Fl female

F2 generation

25% pink-pistil plants, 75% white-pistil plants

Remember our rule about naming the genotype symbol based on the first letter of the recessive condition, in this case pink.

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