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The most obvious outcome of… (Class 10 Science 2023, Q38)

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ScienceCBSE Class 10 · Board exam 2023, set 31/4/1 · 4 marks · Case-based

CBSE Class 10 Science 2023: question 38

Set in the CBSE Class 10 board exam of 2023, paper 31/4/1, 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

The most obvious outcome of the reproductive process is the generation of individuals of similar design, but in sexual reproduction they may not be exactly alike. The resemblances as well as differences are marked. The rules of heredity determine the process by which traits and characteristics are reliably inherited. Many experiments have been done to study the rules of inheritance.

  1. (i)

    Why an offspring of human being is not a true copy of his parents in sexual reproduction ?

    1 mark
  2. (ii)

    While performing experiments on inheritance in plants, what is the difference between and generation ?

    1 mark
  3. (iii)(A)

    Why do we say that variations are useful for the survival of a species over time ?

    2 marks

  4. OR

    (iii)(B)

    Study Mendel's cross between two plants with a pair of contrasting characters: RRYY (Round Yellow) × rryy (Wrinkled Green). He observed 4 types of combinations in generation. Which of these were new combinations ? Why do new features which are not present in the parents, appear in generation ?

    2 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 31/4/1.

Hints

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

Hint 1

How many parents contribute DNA in sexual reproduction?

Hint 2

Recall how Mendel obtained each generation and which traits appeared in each.

Hint 3

For the dihybrid cross, list all four F₂ phenotypes and compare them with the parents.

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. (i) Sexual reproduction involves the fusion of a male gamete and a female gamete, each carrying half the DNA, from two different parents. The offspring gets a new combination of characters from both parents, so variations arise and it is not a true copy of either parent.

    1 mark

  2. (ii) The F₁ generation is the first set of offspring from crossing the two parent plants, and only the dominant trait is expressed in it. The F₂ generation is obtained by self-crossing F₁ plants, and both dominant and recessive traits are expressed in it.

    1 mark

  3. (iii)(A) If the niche of a population changes drastically, for example through a rise in temperature, a population with no variation could be wiped out completely.

    1 mark

  4. If variations are present, some individuals may suit the new conditions. For example, a few heat-resistant bacteria would survive global warming and multiply. So variations help the species survive over time.

    1 mark

  5. (iii)(B) New combinations in F₂: Round green and Wrinkled yellow (besides the parental Round yellow and Wrinkled green, in the ratio 9:3:3:1).

    1 mark

  6. Reason: when two or more traits are involved, their genes are inherited independently of each other (law of independent assortment). They can therefore recombine in ways not present in either parent.

    1 mark

Answer: (i) Fusion of gametes from two parents creates new combinations (variation); (ii) F₁ shows only dominant traits, while F₂ (from selfing F₁) shows both dominant and recessive traits; (iii)(A) variants survive drastic changes in the niche; (iii)(B) round green and wrinkled yellow, due to independent inheritance of traits

Where the marks are

  • Explains the fusion of gametes from two parents leading to variation (1 mark)
  • Gives any one correct difference between F₁ and F₂ (1 mark)
  • Explains survival through variation when the niche changes, e.g. heat-resistant bacteria (2 marks)
  • OR: names the new combinations round green and wrinkled yellow (½ + ½ mark)
  • Gives the reason as independent inheritance of traits (1 mark)

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 paper2023 board paper, set 31/4/1: 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.