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SR.GENETICS.5H BIOLOGY CURRICULUM 2006 PUNNETT SQUARE PRACTICE PROBLEMS A Punnett square is a tool for determining the possible combinations of alleles the offspring of two parents can have. This tool can be used to calculate the probability of having offspring with certain characteristics such as cystic fibrosis or hemophilia. Recall that there are three possible offspring genotypes for each trait because the genotype consists of two alleles, such as AA, Aa, or aa. For dominant traits, however, there are only two phenotypes: either the dominant trait (A) is expressed, or the recessive trait (a) is expressed. Therefore, the ratio of genotypes of offspring will not always be the same as the ratio of phenotypes that will be expressed in those offspring. After you complete these problems, you will be able to use a Punnett square to calculate ratios and percents of the genotypic and phenotypic outcomes of crosses when parent genotypes are known. 1. Draw a Punnett square for a cross of a male whose genotype is Aa with a female whose genotype is aa. Complete the calculations that follow. a. Genotypic ratio homozygous dominant: ___ : heterozygous: ___ : homozygous recessive: ___ simplified ratio: ___ : ___ : ___ b. Phenotypic ratio dominant trait ______ : recessive trait ______ 2. Draw a Punnett square for a cross of a male whose genotype is BB with a female whose genotype is bb. Complete the calculations that follow. a. Genotypic ratio homozygous dominant: ___ : heterozygous: ___ : homozygous recessive: ___ simplified ratio: ___ : ___ : ___ b. Phenotypic ratio dominant trait ______ : recessive trait ______ 711 SR.GENETICS.5H BIOLOGY CURRICULUM 2006 3. Cystic fibrosis is a recessive autosomal genetic disorder that causes thick mucous to be produced by the lungs, liver, and other organs. About one in twenty Caucasians of European ancestry is a carrier for cystic fibrosis. Bill does not have cystic fibrosis and neither does anyone in his family. He wants to have children with Lora. Lora is healthy but her father died from cystic fibrosis. Draw a Punnett square to show possible offspring genotypes of Bill and Lora. Complete the calculations that follow. a. Genotypic ratio homozygous dominant: ___ : heterozygous: ___ : homozygous recessive: ___ simplified ratio: ___ : ___ : ___ b. Phenotypic ratio healthy ______ : cystic fibrosis ______ c. percent chance that Bill and Lora will have a child with CF ____ a carrier ______ 4. Cinnamon hair in cats is recessive to black hair. A black female mated with a black male cat. Her first kitten emerged with cinnamon hair. Draw a Punnett square for a cross of the parent cats. Calculate genotype and phenotype ratios using the same format as in problems 1-3. Explain how the cinnamon kitten could have come from two black cats. Then use your calculations to describe the likelihood that the next kitten will be cinnamon. a. Genotypic ratio ________________________________________________________ b. Phenotypic ratio _______________________________________________________ c. Explanation/Description: ________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 712
