In the snapdragon (dog flower), red flowers (RR) and white flowers (rr) are the two homozygotes…
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In the snapdragon (dog flower), red flowers (RR) and white flowers (rr) are the two homozygotes, and the heterozygote (Rr) is pink. A grower crosses two pink plants and grows offspring. In a second cross, she crosses a pink plant with a white plant and grows offspring.
- In the first cross, how many red, how many pink and how many white plants does she expect?
- In the second cross, how many pink plants does she expect?
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The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. Pink is Rr. First cross Rr × Rr. Second cross Rr × rr. offspring each.
Plan. The phenotype follows the genotype, so use the genotypic ratio.
Step 1. First cross. The ratio is RR : Rr : rr, out of parts. One part is plants. So there are red, pink and white plants.
Step 2. Second cross. Rr gives R or r, and rr gives only r. Half the offspring are Rr (pink) and half are rr (white): pink plants and white plants.
Answer: red, pink and white; pink in the second cross.
Check. In each cross, the numbers add up to .
Watch out. There is no 3 : 1 ratio here. With incomplete dominance the phenotypic ratio is 1 : 2 : 1.
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