Answer the following. (i) Perform a cross between two sickle cell carriers. What ratio is obtained between carrier, disease free and diseased individuals in F1 progeny ? Name the nitrogenous base substituted, in the haemoglobin molecule in this disease. (ii) Explain the difference in inheritance pattern of flower colour in garden pea plant and snap-dragon plant with the help of monohybrid crosses.

BiologyCBSE Class 12 · Board exam 2025, set 57/1/1 · 5 marks · Long answer

CBSE Class 12 Biology 2025: question 33(A)

Set in the CBSE Class 12 board exam of 2025, paper 57/1/1, section E, question 33(A), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q33(A) · 5 marks · Long answerPrinciples of Inheritance and Variation

Answer the following.

(i)
Perform a cross between two sickle cell carriers. What ratio is obtained between carrier, disease free and diseased individuals in F1 progeny ? Name the nitrogenous base substituted, in the haemoglobin molecule in this disease.
0 marks
(ii)
Explain the difference in inheritance pattern of flower colour in garden pea plant and snap-dragon plant with the help of monohybrid crosses.
0 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 57/1/1.

Hints

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

Hint 1

Write each carrier as $H b^{A} H b^{S}$ and draw a Punnett square.

Hint 2

Recall which codon changes in the β-globin gene: GAG becomes GTG.

Hint 3

Compare the F1 phenotype in each plant: is it like one parent, or intermediate between them?

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.

(i) Cross between two sickle-cell carriers: Parents are $H b^{A} H b^{S} \times H b^{A} H b^{S}$ . Gametes from each parent are $H b^{A}$ and $H b^{S}$ .
$H b^{A}$ $H b^{S}$
$H b^{A}$ $H b^{A} H b^{A}$ (disease free) $H b^{A} H b^{S}$ (carrier)
$H b^{S}$ $H b^{A} H b^{S}$ (carrier) $H b^{S} H b^{S}$ (diseased)
½ mark
Ratio in F1: Carrier : Disease free : Diseased = 2 : 1 : 1.
½ mark
Base substituted: In the β-globin gene, the codon GAG changes to GTG, so adenine is substituted by thymine (in mRNA, GAG → GUG, i.e. A replaced by U). As a result, glutamic acid is replaced by valine at the 6th position of the β-globin chain of haemoglobin.
1 mark
(ii) Garden pea (law of dominance): Violet flower colour (V) is completely dominant over white (v). The F1 shows only the dominant parental trait.
½ mark
Pea cross: Violet (VV) × White (vv) gives gametes V and v. F1 is Vv, all violet.
F1 × F1 (Vv × Vv) gives F2 of VV : Vv : vv = 1 : 2 : 1. The phenotypic ratio is 3 violet : 1 white.
1 mark
Snapdragon (incomplete dominance): Red (R) is not completely dominant over white (r). The F1 heterozygote shows an intermediate pink colour that resembles neither parent.
½ mark
Snapdragon cross: Red (RR) × White (rr) gives gametes R and r. F1 is Rr, all pink.
F1 × F1 (Rr × Rr) gives F2 of RR (red) : Rr (pink) : rr (white) = 1 : 2 : 1. Here the phenotypic and genotypic ratios are the same, unlike the 3 : 1 seen in pea.
1 mark

	$H b^{A}$	$H b^{S}$
$H b^{A}$	$H b^{A} H b^{A}$ (disease free)	$H b^{A} H b^{S}$ (carrier)
$H b^{S}$	$H b^{A} H b^{S}$ (carrier)	$H b^{S} H b^{S}$ (diseased)

Answer: (i) Carrier : disease free : diseased = 2 : 1 : 1; adenine is substituted by thymine (GAG→GTG; A→U in mRNA), so glutamic acid is replaced by valine. (ii) Pea shows complete dominance (F1 all violet, F2 3 : 1). Snapdragon shows incomplete dominance (F1 all pink, F2 1 red : 2 pink : 1 white).

Where the marks are

Correct cross $H b^{A} H b^{S} \times H b^{A} H b^{S}$ with offspring genotypes (½ mark)
Ratio carrier : disease free : diseased = 2 : 1 : 1 (½ mark)
Adenine substituted by thymine (GAG→GTG) (1 mark)
Pea follows dominance: violet dominant over white (½ mark)
Correct monohybrid cross in pea, F1 violet, F2 3 : 1 (1 mark)
Snapdragon shows incomplete dominance, F1 pink (½ mark)
Correct monohybrid cross in snapdragon, F2 1 : 2 : 1 (1 mark)

Revise the chapter

Chapter: Principles of Inheritance and Variation (notes, formulas, where marks are lost and every past question)

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 paper2025 board paper, set 57/1/1: every question, by section.

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CBSE Class 12 Biology 2025: question 33(A)

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Step-by-step solution

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