CBSE · Class 12 · Biology · Principles of Inheritance and VariationIn a garden pea plant, tall plant height (T) is dominant over dwarf plant height (t), and purple flower color (P) is dominant over white flower color (p). A dihybrid heterozygous tall and purple flowered plant (TtPp) is test crossed with a double recessive dwarf and white flowered plant (ttpp). Calculate the expected phenotypic and genotypic ratios of the offspring, and show the cross with complete step-by-step working.
Step-by-Step Solution
To determine the offspring of a test cross involving a dihybrid heterozygous individual and a double recessive parent, let us analyze the genotypes step by step.
Step 1: Define the Genotypes of the Parents
- Parent 1 (Dihybrid Heterozygous): TtPp (Tall and Purple flowers)
- Parent 2 (Double Recessive Test Cross Parent): ttpp (Dwarf and White flowers)
Step 2: Determine the Gametes Produced by Each Parent
- Parent 1 (TtPp): Since independent assortment occurs during meiosis, the heterozygous parent produces four types of gametes in equal proportions (25% each):
- TP
- Tp
- tP
- tp
- Parent 2 (ttpp): Being homozygous recessive for both traits, this parent produces only one type of gamete:
- tp
Step 3: Construct the Test Cross Punnett Square\nCrossing the gametes of Parent 1 with the gamete of Parent 2:
| Gametes from Parent 2 (tp) | Gametes from Parent 1 (TP) | Gametes from Parent 1 (Tp) | Gametes from Parent 1 (tP) | Gametes from Parent 1 (tp) |
|---|---|---|---|---|
| tp | TtPp (Tall, Purple) | Ttpp (Tall, White) | ttPp (Dwarf, Purple) | ttpp (Dwarf, White) |
Step 4: Count the Offspring Genotypes and Phenotypes\nFrom the Punnett square, we obtain four distinct genotypes and phenotypes in equal proportions (1:1:1:1 ratio):
- TtPp: Tall and Purple flowers (1/4 or 25%)
- Ttpp: Tall and White flowers (1/4 or 25%)
- ttPp: Dwarf and Purple flowers (1/4 or 25%)
- ttpp: Dwarf and White flowers (1/4 or 25%)
Final Conclusion:
- Phenotypic Ratio: 1 Tall, Purple : 1 Tall, White : 1 Dwarf, Purple : 1 Dwarf, White (i.e., 1:1:1:1)
- Genotypic Ratio: 1 TtPp : 1 Ttpp : 1 ttPp : 1 ttpp (i.e., 1:1:1:1)
💡 Study Guide: This question tests core syllabus concepts from Principles of Inheritance and Variation. For formulas, key summaries, and mock exam reference guides, read the full Principles of Inheritance and Variation Revision Notes.