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How To Use a Punnett Square Predicting Genetic Outcomes QQ#1 Write down anything that comes to mind about Punnett Squares. On a scale from 1-5, how well could you explain/complete a Punnett Square-why (I wont ask you to share this with the class) What Is a Punnett Square? Punnett squares are used to predict the possible outcomes of genetic crosses Only predicts PROBABILITIES, no guarantees! QQ#2 Why might we want to predict the probability of genetic crosses? Setting Up the Punnett Square 1. Determine what possible alleles the parents have. 2. Draw a square, and within the square divide it into 4 equal squares 3. Place the male’s alleles on the top of the Punnett Square. 4. Place the female’s alleles on the left side of the Punnett Square. 5. Like a multiplication table, fill in the offspring possible genotypes Example Example Female is heterozygous for brown hair, meaning… she has one dominant allele (B) and one recessive allele (b). Male is homozygous recessive, meaning… he has two recessive alleles (bb) Example b b B Bb Bb b bb bb Results B b Bb bb Bb bb b Genotypes – 50% Bb 50% bb Phenotypes – 50% brown hair b 50% blonde hair Practice QQ#3 Use a Punnett Square to determine the possible genotypes and phenotypes of the offspring from the following cross. Brown eyes (B) are dominant over blue eyes (b). Cross two heterozygous parents (Bb). Practice answers B b Genotypes 25% BB 50% Bb B BB Bb 25% bb Phenotypes b Bb bb 75% brown eyes 25% blue eyes