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A. In a family of four… (Class 10 Science SQP 2024-25, Q30)

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ScienceCBSE Class 10 · Sample paper, session 2024-25 · 3 marks · Short answer

CBSE Class 10 Science sample paper 2024-25: question 30

From the official CBSE sample question paper for 2024-25, section C, question 30, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q30 · 3 marks · Short answerHeredity

A. In a family of four individuals, the father possessed long ears and the mother possessed short ears. If the parents had pure dominant and recessive traits respectively and F1 individual is married to an unrelated individual of the same genotype, then calculate the ratio of genetic makeup of F2 generation. Show a suitable cross. (2)
B. If father had short ears and the mother had long ears, and both the parents are homozygous for the allelic pair of genes, explain what effect it will have on the ratio of genetic makeup in F2 generation, if F1 individual is married to an unrelated individual of the same genotype. (1)

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

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

Hint 1

Choose a letter: capital for the dominant trait (long), small for recessive (short).

Hint 2

Cross the F1 hybrid with another hybrid using a Punnett square.

Hint 3

Does swapping which parent has the dominant trait change the F1 genotype?

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.

  1. Let L = long ears (dominant) and l = short ears (recessive). Parents: father (pure long) × mother (pure short). Gametes: and . F1: all Ll (long ears).

    1 mark

  2. F1 × same genotype: . Gametes from each parent. Punnett square: , , , . Genotypic ratio of F2 = 1 LL : 2 Ll : 1 ll (phenotypic ratio 3 long : 1 short).

    1 mark

  3. B. Now father × mother . F1 is again all , and again gives 1 LL : 2 Ll : 1 ll. There is no change in the ratio: the cross is still between pure dominant and pure recessive parents, and it does not matter which parent carries which trait.

    1 mark

Answer: A. F2 genotypic ratio 1 LL : 2 Ll : 1 ll; B. No change, still 1 : 2 : 1

Where the marks are

  • Correct parental cross LL × ll giving F1 Ll (1 mark)
  • Ll × Ll gives F2 ratio 1 LL : 2 Ll : 1 ll (1 mark)
  • States the ratio is unchanged with a reason (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Heredity, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2024-25: 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.