Question

In a genetic cross between two pure-line parents differing in the two independently segregating traits, plant height (tall vs dwarf) and flower color (purple vs white), all the F1 plants were tall with purple flowers. In a testcross population involving these F1 individuals, the expected percentage (%) of dwarf plants with purple flowers would be ______ (in integer).

Hint:

Remember in testcrosses involving two traits, calculate the probability of each phenotype by multiplying the independent probabilities of inheriting each trait from a heterozygous F1 individual crossed with a homozygous recessive individual.

Answer 1

Genetics of the Traits:

Since all F1 offspring are tall with purple flowers, both traits (tallness and purple color) are dominant. Assuming Mendelian inheritance, the parental genotypes for height and flower color can be represented as:

• \( T \) (tall) and \( t \) (dwarf)
• \( P \) (purple) and \( p \) (white)

Parental Genotypes:

• Tall parent: \( TT \) or \( TP \)
• Dwarf parent: \( tt \) or \( pp \)

F1 Generation:

All F1 individuals are \( TtPp \) (heterozygous for both traits).

Testcross:

The testcross of F1 individuals (all \( TtPp \)) with the homozygous recessive (dwarf and white) \( ttpp \) results in the following genotype possibilities for F2:

\[ \frac{1}{2} T \text{ (tall)} \times \frac{1}{2} t \text{ (dwarf)} = \frac{1}{4} Tt + \frac{1}{4} tt \] \[ \frac{1}{2} P \text{ (purple)} \times \frac{1}{2} p \text{ (white)} = \frac{1}{4} Pp + \frac{1}{4} pp \]

Combining the probabilities for dwarf (\( tt \)) and purple (\( Pp \) or \( PP \)) gives:

\[ \frac{1}{4} \text{ (dwarf)} \times \frac{1}{2} \text{ (purple)} = \frac{1}{4} \times \frac{1}{2} = \frac{1}{8} \]

However, the correct calculation should consider the probability of \( tt \) and \( Pp \) together:

\[ \frac{1}{4} \text{ (dwarf)} \times \frac{3}{4} \text{ (purple)} = \frac{1}{4} \times \frac{3}{4} = \frac{3}{16} \] Step 3: Correction of Probability Calculation.

The correct expected percentage is calculated as follows:

\[ \text{Percentage} = \frac{1}{4} \text{ (dwarf)} \times \frac{3}{4} \text{ (purple)} \times 100\% = 25\% \] Conclusion:

Explanation: Each trait segregates independently, and the dwarf trait (\( tt \)) combines with the purple trait (\( Pp \) or \( PP \)) to give the desired outcome in \( \frac{1}{4} \) of the cases for color, given that the plant is dwarf, yielding an overall percentage of **25%**.