Answer the following. (a) How is Hardy-Weinberg expression (p² + 2pq + q²) = 1 derived ? (b) List any two factors that disturb the genetic equilibrium.

(a) With p + q = 1, AA = p², aa = q² and Aa = 2pq, so (p+q)² = p² + 2pq + q² = 1; (b) gene flow and genetic drift (also mutation, recombination, natural selection)

BiologyCBSE Class 12 · Board exam 2026, set 57/1/1 · 3 marks · Short answer

CBSE Class 12 Biology 2026: question 24

Set in the CBSE Class 12 board exam of 2026, paper 57/1/1, section C, question 24, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q24 · 3 marks · Short answerEvolution

Answer the following.

(a)
How is Hardy-Weinberg expression $(p^{2} + 2 pq + q^{2}) = 1$ derived ?
2 marks
(b)
List any two factors that disturb the genetic equilibrium.
1 mark

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

Hints

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

Hint 1

Start by giving symbols to the frequencies of the two alleles. What must they add up to?

Hint 2

The probability of two independent events both happening is the product of their probabilities.

Hint 3

For the factors, think of what changes allele frequencies: movement, chance, new alleles, selection.

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.

(a) The Hardy–Weinberg principle states that allele frequencies in a population are stable and stay constant from generation to generation, and that the sum of all allele frequencies is 1.
½ mark
Let one gene have two alleles, A and a, with frequencies $p$ and $q$ . Then $p + q = 1$ .
½ mark
In a diploid individual, the chance of A appearing on both homologous chromosomes is the product of the probabilities, $p \times p = p^{2}$ , which is the frequency of AA. Likewise, the frequency of aa is $q^{2}$ .
½ mark
Heterozygotes Aa can arise in two ways (A from either parent), so their frequency is $2 pq$ . Adding all genotype frequencies gives $(p + q)^{2} = p^{2} + 2 pq + q^{2} = 1$ .
½ mark
(b) Any two factors that disturb genetic equilibrium: gene migration (gene flow) and genetic drift. (Others: mutation, genetic recombination, natural selection.)
1 mark

Answer: (a) With $p + q = 1$ , AA = $p^{2}$ , aa = $q^{2}$ and Aa = $2 pq$ , so $(p + q)^{2} = p^{2} + 2 pq + q^{2} = 1$ ; (b) gene flow and genetic drift (also mutation, recombination, natural selection)

Where the marks are

States that allele frequencies are constant and sum to 1 (½ mark)
$p + q = 1$ for alleles A and a (½ mark)
Frequency of AA is $p^{2}$ and of aa is $q^{2}$ , by the product of probabilities (½ mark)
Aa is $2 pq$ , hence $p^{2} + 2 pq + q^{2} = 1$ (½ mark)
Any two disturbing factors (½ + ½ mark)

Revise the chapter

Chapter: Evolution (notes, formulas, where marks are lost and every past question)
Topic: Hardy-Weinberg Principle

Practise this chapterBoard-style questions from Evolution, one at a time, marked against the answer key and marking points.Back to the full paper2026 board paper, set 57/1/1: 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 24

Hints

Step-by-step solution

Revise the chapter

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