In garden peas, the allele for tall plants is dominant over the allele for short plants. Imagine that a true-breeding tall plant is crossed with a short plant. Then one of their offspring is test crossed. Out of 20 offspring resulting from the test cross, about how many should be tall?

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If the true-breeding tall plant is homozygous dominant (TT) and the short plant is homozygous recessive (tt), their offspring will all be Tt and tall. When one of these Tt plants is test crossed (crossed with a tt plant), the possible genotypes of the offspring are Tt (tall) and tt (short) in a 1:1 ratio. Therefore, out of 20 offspring, we can expect about 10 to be tall (Tt).

In Garden Peas, the Allele for Tall Plants Is Dominant