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

CBSE Class 12 Biology 2023: question 32(b)

Set in the CBSE Class 12 board exam of 2023, paper 57/5/1, 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 answerPrinciples of Inheritance and Variation

It is sometimes observed that the $F_{1}$ progeny has a phenotype that does not resemble either of the two parents and has intermediate phenotype. Explain by taking a suitable example and working out the cross upto $F_{2}$ progeny.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 57/5/1.

Hints

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

Hint 1

Think of a flower in which red crossed with white gives pink.

Hint 2

In this case one allele is not fully dominant over the other. Name the phenomenon and use RR and rr as the parents.

Hint 3

Self the F1 Rr plants with a Punnett square, then compare the phenotypic ratio with the genotypic ratio.

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.

The phenomenon is incomplete dominance. Neither allele is completely dominant over the other, so the heterozygous $F_{1}$ shows a phenotype intermediate between the two parents.
½ mark
Example: flower colour in snapdragon (dog flower, Antirrhinum sp.).
½ mark
Parental cross: pure red $R R$ × pure white $r r$ .
½ mark
Gametes: the red parent forms only $R$ gametes and the white parent forms only $r$ gametes.
½ mark
** $F_{1}$ :** all plants are $R r$ with pink flowers. This intermediate colour resembles neither parent, because one $R$ allele cannot make enough red pigment.
½ mark
** $F_{1}$ selfed:** $R r$ (pink) × $R r$ (pink). Each parent forms $R$ and $r$ gametes in equal proportion.
½ mark
** $F_{2}$ Punnett square:**
♀ \ ♂ $R$ $r$
$R$ $R R$ (Red) $R r$ (Pink)
$r$ $R r$ (Pink) $r r$ (White)
½ mark
** $F_{2}$ progeny:** 1 $R R$ red, 2 $R r$ pink and 1 $r r$ white.
½ mark
Phenotypic ratio: Red : Pink : White = $1 : 2 : 1$ .
½ mark
Genotypic ratio: $R R : R r : r r = 1 : 2 : 1$ . The phenotypic ratio equals the genotypic ratio, unlike Mendel's $3 : 1$ monohybrid ratio. The $R$ and $r$ alleles still segregate unchanged, which shows that the factors do not blend.
½ mark

♀ \ ♂	$R$	$r$
$R$	$R R$ (Red)	$R r$ (Pink)
$r$	$R r$ (Pink)	$r r$ (White)

Answer: Incomplete dominance, e.g. snapdragon flower colour. $R R$ (red) × $r r$ (white) gives $F_{1}$ $R r$ (pink). $F_{2}$ phenotypic and genotypic ratios are both $1 : 2 : 1$ (Red : Pink : White; $R R : R r : r r$ ).

Where the marks are

Names incomplete dominance (½ mark)
Gives snapdragon / Antirrhinum as the example (½ mark)
Parental genotypes RR × rr with phenotypes (½ mark)
Gametes R and r (½ mark)
F1 Rr pink (½ mark)
Selfing of F1 Rr × Rr (½ mark)
F2 gametes / Punnett square (½ mark)
F2 progeny RR red, Rr pink, rr white (½ mark)
Phenotypic ratio 1:2:1 (½ mark)
Genotypic ratio 1:2:1 (½ mark)

Revise the chapter

Chapter: Principles of Inheritance and Variation (notes, formulas, where marks are lost and every past question)

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 57/5/1: every question, by section.

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CBSE Class 12 Biology 2023: question 32(b)

Hints

Step-by-step solution

Revise the chapter

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