Q.64 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 flower
would be _______ (in integer).
In a dihybrid testcross involving F1 tall purple plants from pure-line parents differing in height and flower color, the progeny show a 1:1:1:1 phenotypic ratio due to independent assortment. The expected percentage of dwarf plants with purple flowers is 25%, as this phenotype arises from one of the four equally likely combinations.
Parental Cross and F1 Generation
Pure-line parents represent homozygous states: tall purple (TT PP) crossed with dwarf white (tt pp). All F1 offspring are heterozygous Tt Pp, appearing tall with purple flowers since tall (T) and purple (P) are dominant. This confirms dominance before the testcross.
Testcross Setup
The testcross pairs F1 (Tt Pp) with homozygous recessive tester (tt pp). The tester produces only tp gametes, revealing the F1 gamete contributions. F1 gametes form equally: TP (25%), Tp (25%), tP (25%), tp (25%) due to independent segregation of unlinked genes.
Progeny Phenotypes and Ratios
The Punnett square yields four phenotypes in equal proportions:
| F1 Gamete | Tester Gamete (tp) | Progeny Genotype | Phenotype |
|---|---|---|---|
| TP | tp | Tt Pp | Tall, Purple |
| Tp | tp | Tt pp | Tall, White |
| tP | tp | tt Pp | Dwarf, Purple |
| tp | tp | tt pp | Dwarf, White |
Each class represents 25% of progeny, confirming the 1:1:1:1 ratio for independently assorting traits.
Dwarf Purple Calculation
Dwarf purple plants (tt Pp) result from tP gametes, occurring in 1/4 of offspring. Thus, the percentage is (1/4) × 100 = 25%. No options exist in this fill-in question, but common distractors like 50% (monohybrid error) or 12.5% (F2 confusion) fail here.
CSIR NET Relevance
This tests Mendel’s Law of Independent Assortment in dihybrid testcrosses, key for genetics sections. Deviations signal linkage, but pure 25% confirms independent segregation.
