1. Assume that two pigments, red and blue, mix to give the normal purple color o
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Question
1. Assume that two pigments, red and blue, mix to give the normal purple color of petunia petals. Separate biochemical pathways synthesize the two pigments, as shown below. Each letter represents an enzyme; their corresponding genes are unlinked, and can be symbolized with the same letters.
“White” refers to compounds that are not pigmented, and total lack of pigment will result in a white petal. Red pigment is synthesized from a yellow intermediate that is normally present at a concentration too low to contribute to petal color. Purple flowers result from a mix of blue and red pigment, and green flowers result from a mix of blue and yellow pigment.
Assume that wild-type alleles are dominant and produce functional enzyme; recessive alleles produce non-functional enzyme.
a) What gene or genes would be mutant in a petunia with yellow flowers? Explain briefly.
b) If all other genes are normal, what is the phenotype of an aa mutant flower? If all other genes are normal, what is the phenotype of a bb mutant flower? What is the phenotype of an aa bb double mutant flower? What is the epistatic relationship of the A and B genes (that is, which gene is considered epistatic, and why)?
c) If you crossed two flowers with genotype Aa Bb Cc, what would be the phenotypes of the offspring and in what proportions?
d) If, instead, the A and B loci are genetically linked at a map distance of ten map units, what would be the phenotypes of the offspring and in what proportions for a cross of an aa bb cc tester to an A b C/ a B c individual (the C locus is still unlinked to the other two loci)?
Explanation / Answer
a) The gene C and gene B would be mutant in a petunia with yellow flowers
Gene C product converts white to blue and gene B product coverts yellow to red. So to express only yellow, gene B has to be mutated and presence of blue when yellow is present results in green. So gene C has to be mutated.
b) The phenotype of aa mutant flower would be BLUE.
The phenotype of bb mutant flower would be GREEN
The phenotype of a aabb double mutant flower would be BLUE
B is considered epistatic, since it supresses the product of A
c) 27 purple: 12 Blue: 9 Red: 9 Green: 4 white: 3 yellow
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