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Figures (a) to (d)… (Class 10 Science SQP 2023-24, Q38)

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ScienceCBSE Class 10 · Sample paper, session 2023-24 · 4 marks · Case-based

CBSE Class 10 Science sample paper 2023-24: question 38

From the official CBSE sample question paper for 2023-24, section E, question 38, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q38 · 4 marks · Case-basedCompetency-basedHeredity

Figures (a) to (d) given below represent the type of ear lobes present in a family consisting of 2 children – Rahul, Nisha and their parents. Figure labels: a) Rahul's Father, b) Rahul, c) Rahul's Mother, d) Rahul's sister Nisha. Figures (e) and (f) show 'Type of ear lobes'.

Excited by his observation of different types of ear lobes present in his family, Rahul conducted a survey of the type of ear lobes found {Figure (e) and (f)} in his classmates. He found two types of ear lobes in his classmates as per the frequency given below:

SexFreeAttached
Male3614
Female3119

On the basis of above data answer the following questions.

  1. (a)

    Which of the two characteristics - 'free ear lobe' or 'attached ear lobe' appears to be dominant in this case? Why?

    1 mark
  2. (b)

    Is the inheritance of the free ear lobe linked with sex of the individual? Give reason for your answer.

    1 mark
  3. (c)

    What type of ear lobe is present in father, mother, Rahul and his sister Nisha? Write the genetic constitution of each of these family members which explains the inheritance of this character in this family? (Gene for Free ear lobe is represented by F and gene for attached ear lobe is represented by f for writing the genetic constitution).

    2 marks
  4. (c)

    Suresh's parents have attached ear lobes. What type of ear lobe can be seen in Suresh and his sister Siya? Explain by giving the genetic composition of all.

    2 marks

Figure in the paper: Four photographs of ears labelled a) Rahul's Father, b) Rahul, c) Rahul's Mother, d) Rahul's sister Nisha. Below them, two line drawings labelled (e) and (f) under the caption 'Type of ear lobes': (e) shows an ear lobe that hangs free from the side of the head; (f) shows an ear lobe attached directly to the side of the head. See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2023-24.

Hints

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

Hint 1

Add up the free and attached counts across both sexes. Which trait is more common?

Hint 2

Compare the free-lobe numbers in males and females to decide on sex linkage.

Hint 3

A child showing the recessive trait must be ff. Work backwards to the parents' genotypes.

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. (a) Free ear lobe appears to be dominant. In the survey, of 100 students have free ear lobes and only have attached ones. The trait seen in the large majority is likely to be the dominant one. Also, Rahul has attached lobes although both his parents have free lobes, so attached is recessive.

    1 mark

  2. (b) No, it is not linked with sex. Free ear lobes occur in both males (36 of 50) and females (31 of 50) in nearly equal proportion. In the family too, both the father and Nisha (female) have free lobes. A sex-linked trait would show a clear bias towards one sex.

    1 mark

  3. (c) From the figures: Father has a free ear lobe, Mother has a free ear lobe, Rahul has an attached ear lobe and Nisha has a free ear lobe.

  4. Rahul has attached lobes, so his genotype must be ff. He received one f from each parent. Both parents show the dominant free trait, so each must be Ff: Father Ff, Mother Ff.

    1 mark

  5. Cross gives FF : Ff : ff = 1 : 2 : 1. Nisha has free lobes, so she is Ff (or FF; one F is enough for the free trait). Rahul is ff.

    1 mark

  6. (c) Alternative (Suresh): Both parents have attached ear lobes (recessive), so each is ff. They can pass on only the f allele. Cross: gives only ff.

    1 mark

  7. So Suresh is ff and Siya is ff. Both can only have attached ear lobes.

    1 mark

Answer: (a) Free ear lobe is dominant (found in the majority, 67 of 100). (b) No, it is found in both males and females in similar proportion. (c) Father Ff (free), Mother Ff (free), Rahul ff (attached), Nisha Ff (or FF) (free). OR (c): Father ff, Mother ff; Suresh ff and Siya ff, so both have attached ear lobes.

Where the marks are

  • Free ear lobe dominant, with reason that it occurs in the majority (1 mark)
  • Not sex-linked, with reason that it occurs in both males and females (1 mark)
  • Father Ff, free ear lobe (0.5 mark)
  • Mother Ff, free ear lobe (0.5 mark)
  • Rahul ff, attached ear lobe (0.5 mark)
  • Nisha Ff, free ear lobe (0.5 mark)
  • OR: parents ff, Suresh and Siya ff, all attached; recessive parents can only have recessive children (2 marks)

Revise the chapter

  • Chapter: Heredity (notes, formulas, where marks are lost and every past question)
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 2023-24: 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.