Explain why a homozygous recessive plant (rr) dies as a seedling, and determine the phenotypic ratio of…

Question

Explain why a homozygous recessive plant (rr) dies as a seedling, and determine the phenotypic ratio of offspring from a cross between two heterozygous (Rr) plants.

ScanSolve AI Tutor · Accepted Answer

a) The homozygous recessive plant (genotype rr) is colourless because it lacks chlorophyll, which is essential for photosynthesis. Photosynthesis is the process by which plants produce their own food. While the seedling initially survives on food reserves stored in the seed, these reserves are finite. Once the stored food is depleted, the plant cannot produce new food due to the absence of chlorophyll and therefore dies.

b)

Identify the genotypes of the parents.
The gene for normal chlorophyll is R, and the mutant recessive gene is r.
Pale green plants are heterozygous, meaning their genotype is Rr.
The cross is between two pale green heterozygous plants: $Rr \times Rr$
Determine the possible gametes from each parent.
Each parent (Rr) can produce two types of gametes: R and r.
Use a Punnett square to find the genotypes of the offspring.
$\begin{array}{|c|c|c|} \hline \text{Gametes} & \text{R} & \text{r} \\ \hline \text{R} & \text{RR} & \text{Rr} \\ \hline \text{r} & \text{Rr} & \text{rr} \\ \hline \end{array}$
List the genotypes and corresponding phenotypes of the offspring.
- RR: Normal green plants (1 out of 4 offspring)
- Rr: Pale green plants (2 out of 4 offspring)
- rr: Colourless plants (1 out of 4 offspring). These plants die as seedlings after using up their food reserves.
The types of offspring produced are:
- Normal green (genotype RR)
- Pale green (genotype Rr)
- Colourless (genotype rr)

Explain why a homozygous recessive plant (rr) dies as a seedling, and determine the phenotypic ratio of offspring from a cross between two heterozygous (Rr) plants.

Step-by-step solution

Identify the genotypes of the parents.

Determine the possible gametes from each parent.

Use a Punnett square to find the genotypes of the offspring.

List the genotypes and corresponding phenotypes of the offspring.

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