BiologyCBSE Class 12 · Board exam 2026, set 57/5/1 · 5 marks · Long answer

CBSE Class 12 Biology 2026: question 32(a)

Set in the CBSE Class 12 board exam of 2026, paper 57/5/1, section E, question 32(a), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q32(a) · 5 marks · Long answerPrinciples of Inheritance and Variation

Work out separate monohybrid crosses up to $F_{2}$ generation between two pea plants and two Antirrhinum plants, both having contrasting traits with respect to the colour of the flower. Comment on the patterns of inheritance in the crosses carried out in such two cases.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 57/5/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Write each cross as Parents → Gametes → F₁ → selfing → F₂ Punnett square.

Hint 2

Compare the colour of the F₁ in each plant: does it match one parent or fall in between?

Hint 3

Count the F₂ phenotypes and compare them with the genotypes in each case.

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.

Cross 1 – Pea (flower colour): Parents: violet $V V$ × white $v v$ . Gametes: $V$ and $v$ .
½ mark
$F_{1}$ : all $V v$ , violet. The violet allele masks white. $F_{1}$ is selfed: $V v \times V v$ , giving gametes $V, v$ from each parent.
½ mark
$F_{2}$ Punnett square:
♀ \ ♂ V v
V VV (violet) Vv (violet)
v Vv (violet) vv (white)
½ mark
Phenotypic ratio violet : white = 3 : 1. Genotypic ratio $V V : V v : v v$ = 1 : 2 : 1.
½ mark
Comment: The $F_{1}$ resembles one parent and the $F_{2}$ phenotypic ratio is 3 : 1. This is complete dominance (violet is dominant over white).
½ mark
Cross 2 – Snapdragon (Antirrhinum): Parents: red $R R$ × white $r r$ . Gametes: $R$ and $r$ .
½ mark
$F_{1}$ : all $R r$ , pink. The $F_{1}$ is intermediate and resembles neither parent. $F_{1}$ is selfed: $R r \times R r$ .
½ mark
$F_{2}$ Punnett square:
♀ \ ♂ R r
R RR (red) Rr (pink)
r Rr (pink) rr (white)
½ mark
Phenotypic ratio red : pink : white = 1 : 2 : 1, which is the same as the genotypic ratio.
½ mark
Comment: Neither allele is completely dominant, so the heterozygote shows an intermediate (pink) phenotype. This is incomplete dominance. Here the phenotypic and genotypic ratios are identical, whereas in the pea cross they differ.
½ mark

♀ \ ♂	V	v
V	VV (violet)	Vv (violet)
v	Vv (violet)	vv (white)

♀ \ ♂	R	r
R	RR (red)	Rr (pink)
r	Rr (pink)	rr (white)

Answer: Pea: F₂ violet : white = 3 : 1 (genotypic 1 : 2 : 1) – dominance. Antirrhinum: F₂ red : pink : white = 1 : 2 : 1 – incomplete dominance.

Where the marks are

Pea cross: parents and gametes (½ mark), F₁ violet (½ mark), F₂ checkerboard (½ mark)
Pea: phenotypic ratio 3 : 1 (½ mark); conclusion – dominance (½ mark)
Antirrhinum cross: parents and gametes (½ mark), F₁ pink (½ mark), F₂ checkerboard (½ mark)
Antirrhinum: phenotypic ratio 1 : 2 : 1 (½ mark); conclusion – incomplete dominance (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

Practise this chapterBoard-style questions from Principles of Inheritance and Variation, one at a time, marked against the answer key and marking points.Back to the full paper2026 board paper, set 57/5/1: every question, by section.

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.

CBSE Class 12 Biology 2026: question 32(a)

Hints

Step-by-step solution

Revise the chapter

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