CBSE Class 12 Biology 2023: question 19
Set in the CBSE Class 12 board exam of 2023, paper 57/5/1, section B, question 19, for 2 marks. Try it first; the hints and our step-by-step solution are below.
Q19 · 2 marks · Very short answerPrinciples of Inheritance and Variation
By using Punnett square depict the genotypes and phenotypes of test crosses (where green pod colour (G) is dominant over yellow pod colour (g)) in Garden pea with unknown genotype.
Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 57/5/1.
Hints
Open one hint at a time and try again before the next.
Hint 1
A test cross always uses the homozygous recessive parent, gg.
Hint 2
The unknown green plant could be GG or Gg. Draw a separate Punnett square for each possibility.
Step-by-step solution
Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.
In a test cross, the green-podded plant of unknown genotype is crossed with the homozygous recessive yellow-podded plant (gg).
Case I – unknown plant is homozygous dominant: GG (green) × gg (yellow).
- ½ mark
Punnett square for Case I:
G G g Gg Gg g Gg Gg Genotype: all Gg.
- ½ mark
Phenotype for Case I: all (100%) green pods.
- ½ mark
Case II – unknown plant is heterozygous: Gg (green) × gg (yellow).
G g g Gg gg g Gg gg Genotype: Gg : gg = 1 : 1.
- ½ mark
Phenotype for Case II: 50% green : 50% yellow pods (1 : 1).
Answer: GG × gg → all Gg, all green pods; Gg × gg → 1 Gg : 1 gg, 50% green : 50% yellow pods
Where the marks are
- Punnett square for GG × gg giving all Gg (½ mark)
- States the phenotype as all green pods (½ mark)
- Punnett square for Gg × gg giving Gg : gg in a 1 : 1 ratio (½ mark)
- States the phenotype as 50% green : 50% yellow pods (½ mark)
Revise the chapter
- Chapter: Principles of Inheritance and Variation (notes, formulas, where marks are lost and every past question)
- Topic: Monohybrid Cross and Test Cross
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The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.