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(i) A true breeding tall pea… (Class 12 Biology 2023, Q31b)

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BiologyCBSE Class 12 · Board exam 2023, set 57/1/1 · 5 marks · Long answer

CBSE Class 12 Biology 2023: question 31(b)

Set in the CBSE Class 12 board exam of 2023, paper 57/1/1, section E, question 31(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q31(b) · 5 marks · Long answerPrinciples of Inheritance and Variation

(i) A true breeding tall pea plant with round seeds is crossed with a recessive dwarf pea plant having wrinkled seeds. Work out the cross up to F2 generation giving the phenotypic ratios of F1 and F2 generation respectively.
(ii) State the Mendelian principle that can be derived only with the help of such a cross.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 57/1/1.

Hints

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

Hint 1

A true-breeding dominant parent is homozygous for both genes; the recessive parent is homozygous recessive.

Hint 2

An F₁ dihybrid gives four kinds of gametes in equal proportion.

Hint 3

Which Mendel's law can only be shown when two characters are followed together?

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) Parents: Tall round TTRR × Dwarf wrinkled ttrr

    ½ mark

  2. Gametes: TR × tr

    ½ mark

  3. F₁: TtRr, all tall with round seeds. F₁ phenotypic ratio: 100% tall round (1 : 0).

    ½ mark

  4. Selfing of F₁: TtRr × TtRr

    ½ mark

  5. Gametes from each parent: TR, Tr, tR, tr. Punnett square:

    ♂ \ ♀TRTrtRtr
    TRTTRRTTRrTtRRTtRr
    TrTTRrTTrrTtRrTtrr
    tRTtRRTtRrttRRttRr
    trTtRrTtrrttRrttrr
    2 marks

  6. F₂ phenotypes: Tall round 9 : Tall wrinkled 3 : Dwarf round 3 : Dwarf wrinkled 1, i.e. 9 : 3 : 3 : 1.

    ½ mark

  7. (ii) Law of Independent Assortment: when two pairs of traits are combined in a hybrid, the segregation of one pair of characters is independent of the other pair.

    ½ mark

Answer: F₁: all tall round (TtRr); F₂: tall round : tall wrinkled : dwarf round : dwarf wrinkled = 9 : 3 : 3 : 1; Law of Independent Assortment

Where the marks are

  • Parental genotypes TTRR × ttrr (½ mark)
  • Parental gametes TR and tr (½ mark)
  • F₁ TtRr, all tall round (½ mark)
  • Selfing of F₁ shown (½ mark)
  • Four types of gametes with correct 4 × 4 Punnett square (2 marks)
  • F₂ phenotypic ratio 9 : 3 : 3 : 1 (½ mark)
  • States the law of independent assortment (½ mark)

Revise the chapter

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 paper2023 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.