AP Biology : Understanding Punnett Squares and Test Crosses

Study concepts, example questions & explanations for AP Biology

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Example Questions

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Example Question #61 : Inheritance

A new type of plant is shown to have two distinct traits for its seeds: seed color and seed shape. Green color is dominant to white, and a long shape is dominant to round. If two plants heterozygous for both traits were crossed, what is the probability that an offspring would show the dominant phenotype for both traits?

Possible Answers:

Correct answer:


This question requires that we do a dihybrid cross. The cross in question is AaBb x AaBb, using A to represent dominant green color and B to represent dominant long shape. The parents are heterozygous for both traits, meaning they will carry one dominant color allele and one dominant shape allele.

The result of a punnet square for a dihybrid cross is: 1 AABB, 3 Aabb, 8 AaBb, 3 aaBb, 1 aabb.

This gives a total of sixteen different offspring. Two different genotypes carry dominant alleles for both traits: AABB and AaBb. There are a total of nine offspring between these two genotypes. The probability of an offspring being dominant for both traits is thus nine out of sixteen.

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