Do the alleles for different characters always sort independently?
In biology lab, you conduct a breeding experiment to test Mendel’s law of independent assortment. You study two characters in a new plant species recently discovered on campus:
Flower color, which can be blue (BB) or purple (bb)
Petal shape, which can be pointy (PP) or rounded (pp)
You use the following procedure.
In the parental generation, you breed a plant that you know to be homozygous for blue-pointy flowers (BBPP) with a plant that you know to be homozygous for purple-rounded flowers (bbpp).
In the F1 generation, all your plants have blue-pointy flowers (BbPp).
You then allow the F1 plants to self-pollinate to produce F2 offspring. In the F2 generation, you obtain 80 plants with the following phenotypes. Note that an underscore “_” in the genotype indicates that the second allele for that gene could be either dominant or recessive:
Phenotype Number of individuals
Blue flower/pointy petal (B_P_) 59
Blue flower/rounded petal (B_pp) 1
Purple flower/pointy petal (bbP_) 0
Purple flower/rounded petal (bbpp) 20
To try to explain this unusual data, you come up with two alternate hypotheses in addition to your original hypothesis of independent assortment.
Hypothesis 1: The alleles for flower color and petal shape are found on different chromosomes. (This is independent assortment as observed by Mendel with the characters of seed color and shape.)
Hypothesis 2: The alleles for flower color and petal shape are found on different chromosomes, but the blue-rounded (B_pp) and purple-pointy (bbP_) phenotypes typically do not survive, for a reason that has yet to be determined.
Hypothesis 3: The alleles for flower color and petal shape are found close to each other on the same chromosome.
Drag the labels to their appropriate locations in the table below. Labels may be used more than once. (Hint: First, figure out the predicted F1 gametes for each hypothesis; then construct a Punnett square to help you fill in the rest of the table.)
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