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Name____________________________ Punnett Squares: A Dihybrid Cross A Dihybrid Cross traces two traits from parents to offspring. Example 1: In pea plants, tall stems are dominant over short stems; and Axial flowers are dominant over Terminal flowers. Two plants are crossbred. The first parent is heterozygous for stem height and heterozygous for flower type. The second parent is also hybrid for both of these traits. What genotypes and phenotypes can be expected in the offspring? a. Step One: Write down gene symbols and what they stand for. b. Step Two: Write down the parent genotypes. c. Step Three: Complete the Punnett Square (use the one provided below). d. Step Four: Summarize the genotype and phenotype results. Genotype and Phenotype Results of Cross: Example 2: (On your own) In cats, black fur is dominant over white fur and short tails are dominant over long tails. A heterozygous black fur cat with a long tail is mated with a homozygous black fur cat, which has a heterozygous short tail. What genotypes and phenotypes can be expected in the offspring? Name_____ANSWER KEY________ Punnett Squares: A Dihybrid Cross A Dihybrid Cross traces two traits from parents to offspring. Example 1: In pea plants, tall stems are dominant over short stems; and Axial flowers are dominant over Terminal flowers. Two plants are crossbred. The first parent is heterozygous for stem height and heterozygous for flower type. The second parent is also hybrid for both of these traits. What genotypes and phenotypes can be expected in the offspring? a. Step One: Write down gene symbols and what they stand for. A = Axial a = terminal T = Tall t = short b. Step Two: Write down the parent genotypes. Parent 1 = AaTt Parent 2 = AaTt c. Step Three: Complete the Punnett Square (use the one provided below). d. Step Four: Summarize the genotype and phenotype results. AT At aT at AT At aT at AATT AATt AaTT AaTt AATt AAtt AaTt AAtt AaTT AaTt aaTT aaTt AaTt Aatt aaTt aatt Genotype and Phenotype Results of Cross: Genotype AATT 1/16 Phenotype Axial, tall 9/16 AATt AAtt AaTT AaTt Aatt aaTT aaTt aatt 2/16 1/16 2/16 4/16 2/16 1/16 2/16 1/16 Axial, short 3/16 Terminal, tall 3/16 Terminal, short 1/16 Phenotypic ratio 9:3:3:1 Example 2: (On your own) In cats, black fur is dominant over white fur and short tails are dominant over long tails. A heterozygous black fur cat with a long tail is mated with a homozygous black fur cat, which has a heterozygous short tail. What genotypes and phenotypes can be expected in the offspring? Parent 1 = Bbss Parent 2 = BBSs BS Bs BS Bs Genotype BBSs 4/16 BBss 4/16 BbSs 4/16 Bbss 4/16 Bs BBSs BBss BBSs BBss Bs BBSs BBss BBSs BBss bs BbSs Bbss BbSs Bbss bs BbSs Bbss BbSs Bbss Phenotype Black, short ½ Black, long ½ Black, short Black, long
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