I don't get this problem, at all.
Assume that two pigments, red and blue, mix to give the normal purple color of petunia petal. Separate biochemical pathways synthesize the two pigments, as shown in the top two rows of the accompanying diagram. "White" refers to compounds that are not pigments. (Total lack of pigment results in a white petal.) Red pigment forms from a yellow intermediate that is normally at a concentration too low to color petals.
Pathway I ... → white1 →E blue
Pathway II ... → white2→A yellow→B red
C↑
Pathway III ... →white3 →D white4
( the letters should be on top of the arrows, and the numbers are subscripts)
A third pathway, whose compounds do not contribute pigment to petals, normally does not affect the blue and red pathways, but, if one of its intermediates(white) should build up in concentration, it can be converted into the yellow intermediate of the red pathway. In the diagram, the letters A through E represent enzymes; their corresponding genes, all of which are unlinked, may be symbolized by the same letters. Assume that wild-type alleles are dominant and encode enzyme function and that recessive alleles result in a lack of enzyme function. Deduce which combinations of true-breeding parental genotypes could be crossed to produce F2 progeny in the following ratios:
a. 9 purple: 3 green: 4 blue b. 9 purple: 3 red: 3 blue: 1 white c. 13 purple: 3 blue d. 9 purple: 3 red: 3 green: 1 yellow
(Note: No mutations, blue mixed with yellow makes green)
Please help!
Well, it sounds like you're mixing more than just pigments in this problem! But don't worry, I'll try to break it down for you with my clownish humor.
First, let's look at pathway III. It says that compounds in this pathway don't contribute pigment to petals - they're just there to confuse us, I guess. If one of these compounds builds up (let's call it white), it can be converted into the yellow intermediate of the red pathway. Sneaky, right?
Now, let's tackle the parent genotypes that could produce the F2 progeny ratios.
a. 9 purple: 3 green: 4 blue - To get this ratio, we need a cross between a purple parent (let's call it P) and a blue parent (let's call it B). So, the genotypes could be PP x BB. Simple enough!
b. 9 purple: 3 red: 3 blue: 1 white - Here, we need to take into account that white can be converted into yellow of the red pathway. So, we need a cross between a purple parent (P) and a blue parent (B). But we also need a white parent (W) to make things interesting. The genotypes could be PW x PB.
c. 13 purple: 3 blue - This one is a bit tricky because we don't have any green or red in the ratio. So, let's keep it simple and say we just need a cross between two purple parents (PP x PP). No funny business here!
d. 9 purple: 3 red: 3 green: 1 yellow - Ah, the infamous yellow appears! To get this ratio, we need a cross between a purple parent (P) and a green parent (G). And of course, we also need a red parent (R) to make things colorful. The genotypes could be PR x PG.
I hope this helps, and remember, when in doubt, always add some clownish humor to lighten the mood! Good luck!
To determine which combinations of true-breeding parental genotypes could produce the given ratios of F2 progeny, we need to consider the inheritance pattern of the pigments and enzymes involved in the pathways.
Let's analyze each given ratio and deduce the possible parental genotypes:
a. 9 purple: 3 green: 4 blue
To get a phenotypic ratio of 9 purple: 3 green: 4 blue, we need to have a dominant allele for purple pigment, a recessive allele for green pigment, and a dominant allele for blue pigment.
Possible parental genotypes:
- Purple parent: At least one dominant allele for purple pigment (AA or Aa).
- Green parent: Both recessive alleles for green pigment (aa).
- Blue parent: At least one dominant allele for blue pigment (BB or Bb).
Cross between Purple (A_/B_) and Green (aa/B_): AaBb x aabb
- A_ (dominant allele for purple) is required for purple pigment.
- B_ (dominant allele for blue) is required for blue pigment.
- aa (recessive genotype for green) will result in no green pigment.
- The F1 generation will all be purple (AaBb).
F1 purple (AaBb) x F1 purple (AaBb): AaBb x AaBb
- This cross can produce the required ratio of 9 purple:3 green:4 blue in the F2 generation.
b. 9 purple: 3 red: 3 blue: 1 white
To get a phenotypic ratio of 9 purple: 3 red: 3 blue: 1 white, we need to have a dominant allele for purple and blue pigments, a recessive allele for red pigment, and a double recessive allele for white pigment.
Possible parental genotypes:
- Purple parent: At least one dominant allele for purple pigment (AA or Aa).
- Red parent: Both recessive alleles for red pigment (aa).
- Blue parent: At least one dominant allele for blue pigment (BB or Bb).
- White parent: Both double recessive alleles for white pigment (cc).
Cross between Purple (A_/B_) and Red (aa/B_) and Blue (B_/cc): AaBb x aacc
- A_ (dominant allele for purple) is required for purple pigment.
- B_ (dominant allele for blue) is required for blue pigment.
- aa (recessive genotype for red) will result in no red pigment.
- cc (double recessive genotype for white) will result in white pigment.
- The F1 generation will be purple (AaBbcc).
F1 purple (AaBbcc) x F1 purple (AaBbcc): AaBbcc x AaBbcc
- This cross can produce the required ratio of 9 purple:3 red:3 blue:1 white in the F2 generation.
c. 13 purple: 3 blue
To get a phenotypic ratio of 13 purple: 3 blue, we need to have a dominant allele for purple pigment and a dominant allele for blue pigment.
Possible parental genotypes:
- Purple parent: At least one dominant allele for purple pigment (AA or Aa).
- Blue parent: At least one dominant allele for blue pigment (BB or Bb).
Cross between Purple (A_/B_) and Blue (B_/cc): AaBb x Bbcc
- A_ (dominant allele for purple) is required for purple pigment.
- B_ (dominant allele for blue) is required for blue pigment.
- The F1 generation will all be purple (AaBb).
F1 purple (AaBb) x F1 purple (AaBb): AaBb x AaBb
- This cross can produce the required ratio of 13 purple:3 blue in the F2 generation.
d. 9 purple: 3 red: 3 green: 1 yellow
To get a phenotypic ratio of 9 purple: 3 red: 3 green: 1 yellow, we need to have a dominant allele for purple and green pigments, a recessive allele for red pigment, and a double recessive allele for yellow pigment.
Possible parental genotypes:
- Purple parent: At least one dominant allele for purple pigment (AA or Aa).
- Red parent: Both recessive alleles for red pigment (aa).
- Green parent: At least one dominant allele for green pigment (GG or Gg).
- Yellow parent: Both double recessive alleles for yellow pigment (gg).
Cross between Purple (A_/B_) and Red (aa/B_) and Green (G_/gg): AaBb x AAgg
- A_ (dominant allele for purple) is required for purple pigment.
- aa (recessive genotype for red) will result in no red pigment.
- G_ (dominant allele for green) is required for green pigment.
- gg (double recessive genotype for yellow) will result in yellow pigment.
- The F1 generation will be purple (AaBbGg).
F1 purple (AaBbGg) x F1 purple (AaBbGg): AaBbGg x AaBbGg
- This cross can produce the required ratio of 9 purple:3 red:3 green:1 yellow in the F2 generation.
Remember that these are potential crosses that could give rise to the specified ratios. Other combinations may also be possible, but these are the most likely possibilities based on the given information.