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In shorthorn cattle… (Class 12 Biology SQP 2023-24, Q32b)

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BiologyCBSE Class 12 · Sample paper, session 2023-24 · 5 marks · Long answer

CBSE Class 12 Biology sample paper 2023-24: question 32(b)

From the official CBSE sample question paper for 2023-24, section E, question 32(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q32(b) · 5 marks · Long answerCompetency-basedPrinciples of Inheritance and Variation

In shorthorn cattle, the coat colours red or white are controlled by a single pair of alleles. A calf which receives the allele for red coat from its mother and the allele for white coat from its father is called a 'roan'. It has an equal number of red and white hairs in its coat.
a) Is this an example of codominance or of incomplete dominance?
b) Give a reason for your answer.
c) With the help of genetic cross explain what will be the consequent phenotype of the calf when
i. red is dominant over white
ii. red is incompletely dominant.

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

Look closely at the roan coat: are the hairs blended in colour, or are the red and white hairs separate?

Hint 2

Write the calf's genotype from the alleles it got from each parent, then think what that heterozygote looks like under complete and under incomplete dominance.

Hint 3

Cross two heterozygotes in a Punnett square to get the F ratios.

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) The roan calf shows codominance.

    ½ mark

  2. (b) Reason: The roan coat has red hairs and white hairs side by side, in equal numbers. No hair is an intermediate (pink) colour. Both alleles are therefore fully and separately expressed in the heterozygote (Rr), and neither hides the other. This is the definition of codominance.

    ½ mark

  3. **(c)(i) Red () completely dominant over white ():**

    Parents: (red, mother) (white, father)

    Gametes: and

    F (the calf): . All F calves are red, because the dominant allele masks and the phenotype is the same as the red parent.

    1 mark

  4. Selfing F () to show the full pattern:

    red red
    red white

    F phenotypic ratio is red : white = 3 : 1. The genotypic ratio is = 1 : 2 : 1.

    1 mark

  5. (c)(ii) Red incompletely dominant over white:

    Parents: (red) (white)

    Gametes: and

    F (the calf): . All F calves are pink, an intermediate shade, because one allele cannot produce enough pigment for a full red coat.

    1 mark

  6. Crossing F ():

    red pink
    pink white

    F ratio is red : pink : white = 1 : 2 : 1. The phenotypic ratio equals the genotypic ratio.

    1 mark

Answer: (a) Codominance. (b) Both red and white hairs appear separately in the heterozygote, with no blending. (c)(i) Calf is red; F red : white = 3 : 1. (ii) Calf is pink (intermediate); F red : pink : white = 1 : 2 : 1.

Where the marks are

  • Identifies codominance (0.5 mark)
  • Reason: both alleles expressed independently as separate red and white hairs, no intermediate colour (0.5 mark)
  • Complete dominance cross RR × rr giving Rr red calf (1 mark)
  • F2 Punnett square with 3 : 1 ratio (1 mark)
  • Incomplete dominance cross giving Rr pink calf (1 mark)
  • F2 Punnett square with 1 : 2 : 1 ratio (1 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 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.