ACT Preparation / Unit 22 ACT 2106 worked example, 60 to 90 seconds
Predicting Offspring With a Punnett Square
The idea
A cross is predicted in three steps. First, write each parent's two alleles, using a capital letter for the dominant allele and a lowercase letter for the recessive. Second, draw a two-by-two grid with one parent's alleles across the top and the other's down the side, and fill each box with one allele from each. Third, count the boxes: each is one quarter of the expected offspring, and the dominant trait appears wherever a capital letter does.
In practice
Worked example: two beetles, each heterozygous, Bb, for black wings over red. The grid gives BB, Bb, Bb and bb. Three of the four boxes contain B, so about three quarters of the offspring are expected to be black and one quarter red, a ratio of 3 to 1. Crossing Bb with bb gives Bb, Bb, bb and bb: half black, half red.
Where people slip
The ratios are expected proportions, not guarantees. A family of four can show any mix, much as four coin flips can all land heads.
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