A florist has a pea plant with violet flowers…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A florist has a pea plant with violet flowers. Violet (V) is dominant over white (v). She does not know whether the plant is VV or Vv, so she crosses it with a plant that has white flowers. The cross gives plants with violet flowers and plants with white flowers.
- What is the genotype of the violet parent?
- What name is given to this cross, and why must the white parent be used?
- What would the offspring have looked like if the violet parent had been VV?
Need a reminder? The key ideaShow
In a monohybrid cross with complete dominance, the F1 of TT × tt is all Tt and tall. The F2 has the genotypic ratio 1 TT : 2 Tt : 1 tt and the phenotypic ratio 3 tall : 1 dwarf. A test cross uses a homozygous recessive partner (tt). A Tt parent gives T and t with equal chance, so each child of two carriers has a chance of of showing the recessive trait.
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. Violet parent × white parent. Offspring: violet and white.
Plan. Compare the offspring with what each possible parent would give.
Step 1. (a) The white parent is vv and gives only v. The white offspring (vv) received v from the violet parent too, so the violet parent carries v. It is Vv. The ratio is close to .
Step 2. (b) This is a test cross. The recessive parent gives only the recessive allele, so each offspring shows which allele it got from the plant under test. A heterozygous partner would hide this.
Step 3. (c) A VV plant gives only V gametes, so every offspring would be Vv with violet flowers. There would be no white plants.
Answer: (a) Vv; (b) a test cross, with a homozygous recessive partner; (c) all violet.
Check. A Vv × vv cross gives Vv and vv in equal numbers, so about half are violet.
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