Name: ______
Date: ______Block: ______
DIHYBRID CROSS PRACTICE:
How to complete a genetic cross that involve two traits
Step 1: Develop a key
Step 2: Write down “cross”
Step 3: Determine each parent’s gametes
Step 4: Set-up and complete the punnett square
Step 5: Analyze the punnett square genotypes
Step 6: Analyze the punnett square phenotypes
In llamas, floppy ears are dominant to pointy ears. Also in llamas, bowlegs are recessive.
1. Develop a "key" showing the letters for the two traits. (Each trait should have a different letter). Show the phenotypes of your letters too. Ex: FF = floppy ears. LL = normal legs
a. A llama with floppy ears (Ff) and bowlegs is crossed with one that is heterozygous for both traits. Show the genotypes of these two parents
i. Draw a punnet square showing the resulting offspring
ii. Show the ratios of the resulting phenotypes, use fractions.
b. A llama that has pointy ears and bowlegs is crossed with one that is homozygous dominant for both traits. Show the genotypes of the parents.
i. Draw a punnet square showing the resulting offspring
ii. Show the ratios of the resulting phenotypes, use fractions
c. A llama that is heterozygous for both traits is crossed with another llama that is heterozygous for both traits. Show the genotypes of the two parents.
i. Draw a punnet square showing the resulting offspring.
ii. Show the ratios of the resulting phenotypes, use fractions.
Question #1
A. Using the following individual as P1, use a Punnett square to determine the genotypic & phenotypic ratios for the F1 and F2 generations. Round beaks are dominant to flat beaks.
A true breeding flat beaked bird is mated to a true breeding round beaked bird.
B. Our 10 year study using these two birds for the parental group yields the following phenotypic ratios for F2 birds. 190 birds have round beaks and 62 develop flat beaks. Why do we need to testcross some of these birds? How many birds must we do testcrosses for? Show the set up of one testcross Punnett square.
C. The testcross for one of the round beaked birds produces 10 round beaked birds and 1 flat beaked bird. What is the genotype for this round beaked bird? Explain your answer.
Question #2
A. Now we want to add another trait to follow. This trait is toe nail color. Black toe nails are dominant to clear. Using the following individuals as the P1, use a Punnett square to determine the genotypic and phenotypic ratios for the F1 and F2 generations.
A true breeding flat beaked clear toe nailed bird is mated to a true breeding round beaked black toe nailed bird.
B. If the F2 generation has 712 birds, approximately how many birds should have round beaks and clear toe nails?
C. What is the genotype and phenotype of a bird that should be used in a testcross to help determine the unknown genotype