Question

A tall round seeded plant (TTRR) is crossed with a dwarf wrinkled seeded plant (ttrr).
(a) Write the genotype and phenotype of F$_1$ plants.
(b) If F$_1$ plants are self-pollinated, write the possible phenotypes in F$_2$.

Hint:

Remember: In a dihybrid cross of the type TtRr $\times$ TtRr, the F$_2$ phenotypic ratio is always 9:3:3:1 if the traits assort independently.

Answer 1

Step 1: Identify the dominant and recessive traits.
In this question, T represents the dominant trait for tallness and t represents the recessive trait for dwarfness. Similarly, R represents the dominant trait for round seeds and r represents the recessive trait for wrinkled seeds.
Step 2: Write the parental cross.
The given parental cross is:
\[ TTRR \times ttrr \] The first parent can produce only one type of gamete, that is TR, and the second parent can also produce only one type of gamete, that is tr.
Step 3: Determine the F$_1$ genotype and phenotype.
When gametes TR and tr combine, all F$_1$ offspring will have the genotype:
\[ TtRr \] Since both T and R are dominant, all F$_1$ plants will show the phenotype tall round seeded.
Step 4: Self-pollination of F$_1$ plants.
Now the F$_1$ plants are self-pollinated:
\[ TtRr \times TtRr \] This is a typical dihybrid cross. In the F$_2$ generation, the characters for height and seed shape assort independently, producing different phenotypic combinations.
Step 5: Write the possible phenotypes in F$_2$.
The possible phenotypes obtained in the F$_2$ generation are:

• Tall round seeded plants
  
• Tall wrinkled seeded plants
  
• Dwarf round seeded plants
  
• Dwarf wrinkled seeded plants
  

Their phenotypic ratio is:
\[ 9 : 3 : 3 : 1 \]