Each plant was crossed with normal (leafy) pure-bred plants to generate the foll
ID: 11930 • Letter: E
Question
Each plant was crossed with normal (leafy) pure-bred plants to generate the following phenotypic ratios:
F1 phenotype F2 phenotype
Line 1 x leafy all leves 3:1 leaves, no leaves
Line 2 x leafy all no leaves 3:1 no leaves, leaves
Line 1 X line 2 all no leaves 13:3 no leaves, leaves
Why do two apparently similar plant lines produce such different results? For each cross depicted in the table, show the genotypes of all parental, F1, and F2 plants. Show the results of a total of three crosses.
Explanation / Answer
CROSS #1:
F1 phenotype F2 phenotype
Line 1 x leafy all leves 3:1 leaves, no leaves
Line 2 x leafy all no leaves 3:1 no leaves, leaves
Line 1 X line 2 all no leaves 13:3 no leaves, leaves
LET R be "with leaves" and r be "no leaves"
We can tell that the parents were RR x Rr because that's the only combination that produces all leaves, but whose offspring - "tainted" with that one lowercase r - can produce a 3:1 ratio.
So, parents: RR and Rr
Next, we have our F1 generation. The only combination that can produce a 3:1 ratio would be Rr x Rr, so our F1 generation = Rr and Rr
More specifically, our F1 generation has 50% Rr and 50% RR
Next, we have our F2 generation.
We see that they have a 3:1 ratio of leaves and no leaves.
F2 Generation = 25% rr (no leaves), 50% Rr (leaves), and 25% RR (leaves)
CROSS #2:
F1 phenotype F2 phenotype
Line 1 x leafy all leves 3:1 leaves, no leaves
Line 2 x leafy all no leaves 3:1 no leaves, leaves
Line 1 X line 2 all no leaves 13:3 no leaves, leaves
Parents: here, our parents produced no leaves in their kids (F1 = parents' kids) but since they have leaves in F2, they can't be all leafless (aka they can't both be rr), so our parents must be Rr and rr
F1: Since no one has leaves, the genotype is 100% rr
F3: We see that the F2 phenotype is 3:1 no leaves, leaves, so our only option is
75% rr and 25% Rr (this is the 1/4th that has leaves)
CROSS #3:
phenotype F2 phenotype
Line 1 x leafy all leves 3:1 leaves, no leaves
Line 2 x leafy all no leaves 3:1 no leaves, leaves
Line 1 X line 2 all no leaves 13:3 no leaves, leaves
We see that these parents produced all no leaves, so each parent must have an r gene. Next, we see that 3/16 of the grandkids have leaves.
Therefore, we're crossing Rr x rr in the parent generation
This gives us a 50% Rr and 50% rr in the F1 generation
& 3/16 Rr and 13/16 rr in the F2 generation
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