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Biology LibreTexts

9.2: Stating the Hypothesis - Which Phenotype Is Dominant?

  • Page ID
    24179
  • In this activity you will examine the offspring of a genetic cross between a stickleback from the ocean (marine stickleback) and a freshwater stickleback from Bear Paw Lake.

    1. Based on what you learned in the film, what are the phenotypes of these two parental stickleback fish?
      Marine: were pelvic spines present, or were pelvic spines absent?
      Bear Paw Lake: were pelvic spines present, or were pelvic spines absent?
    2. If we start with the simplest assumption that the presence or absence of pelvic spines is controlled by a single gene with two alleles, how would you denote the genotype of the two homozygous parents?
    3. What is your hypothesis for which phenotype is dominant and which one is recessive?
    4. Based on your hypothesis, what would you expect to be the results of the cross between the marine and Bear Paw Lake stickleback parents? Make your prediction using a Punnett square.
    5. What would be the ratio of stickleback with pelvic spines to stickleback without spines in the first filial (F1) generation?
    6. Using the Punnett square below, what do you expect would be the result of crossing two F1 fish to produce the second filial (F2) generation?
    7. What would be the ratio of stickleback with pelvic spines to stickleback without spines in the F2 generation?
    8. If you had 40 offspring in the F2 generation, approximately how many would you expect to have pelvic spines and how many to lack pelvic spines?