Mendel worked out the rules of heredity by working on garden pea using a number of visible contrasting characters. He conducted several experiments by making a cross with one or two pairs of contrasting characters of pea plant. On the basis of his observations he gave some interpretations which helped to study the mechanism of inheritance. (i) When Mendel crossed pea plants with pure tall and pure short characteristics to produce F_1 progeny, which two observations were made by him in F_1 plants? (ii) Write one difference between dominant and recessive trait. (iii)(A) In a cross with two pairs of contrasting characters RRYY × rryy (Round Yellow) (Wrinkled Green) Mendel observed 4 types of combinations in F_2 generation. By which method did he obtain F_2 generation? Write the ratio of the parental combinations obtained and what conclusions were drawn from this experiment. (iii)(B) Justify the statement: "It is possible that a trait is inherited but may not be expressed."

ScienceCBSE Class 10 · Board exam 2024, set 31/1/1 · 4 marks · Case-based

CBSE Class 10 Science 2024: question 38

Set in the CBSE Class 10 board exam of 2024, paper 31/1/1, 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

Mendel worked out the rules of heredity by working on garden pea using a number of visible contrasting characters. He conducted several experiments by making a cross with one or two pairs of contrasting characters of pea plant. On the basis of his observations he gave some interpretations which helped to study the mechanism of inheritance.

(i)
When Mendel crossed pea plants with pure tall and pure short characteristics to produce $F_{1}$ progeny, which two observations were made by him in $F_{1}$ plants?
1 mark
(ii)
Write one difference between dominant and recessive trait.
1 mark
(iii)(A)
In a cross with two pairs of contrasting characters
$R R Y Y \times r r y y$
(Round Yellow) (Wrinkled Green)
Mendel observed 4 types of combinations in $F_{2}$ generation. By which method did he obtain $F_{2}$ generation? Write the ratio of the parental combinations obtained and what conclusions were drawn from this experiment.
2 marks
OR
(iii)(B)
Justify the statement: "It is possible that a trait is inherited but may not be expressed."
2 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 31/1/1.

Hints

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

Hint 1

Think about what the F₁ plants looked like compared with both parents.

Hint 2

The dihybrid F₂ ratio is 9:3:3:1 — pick out only the combinations that match the parents.

Hint 3

For (iii)(B), recall what happens to shortness in F₁ and in F₂ of a monohybrid cross.

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.

(i) Observation 1: all the $F_{1}$ plants were tall; no short plants appeared.
½ mark
Observation 2: no plants of medium height appeared, so there was no blending; only the dominant parental trait (tallness) was seen.
½ mark
(ii) A dominant trait is expressed even when only a single copy of its factor is present (e.g. Tt is tall), whereas a recessive trait is expressed only when both copies are recessive (e.g. tt is short).
1 mark
(iii)(A) Method: the $F_{2}$ generation was obtained by self-pollination of the $F_{1}$ plants (RrYy).
½ mark
$F_{2}$ ratio was Round Yellow : Round Green : Wrinkled Yellow : Wrinkled Green = 9 : 3 : 3 : 1. The parental combinations Round Yellow : Wrinkled Green = 9 : 1.
½ mark
Conclusion: new combinations (round green, wrinkled yellow) appeared, so the two traits (seed shape and seed colour) are inherited independently of each other.
1 mark
(iii)(B) When pure tall (TT) pea plants are crossed with pure short (tt) plants, all $F_{1}$ plants (Tt) are tall, so shortness is not expressed although the factor 't' is present in them.
1 mark
On self-pollinating $F_{1}$ , short plants (tt) reappear in $F_{2}$ in the ratio tall : short = 3 : 1. This shows the recessive trait was inherited by $F_{1}$ but stayed unexpressed, so a trait may be inherited without being expressed.
1 mark

Answer: (i) All F₁ tall; no intermediate height. (ii) Dominant expressed with one copy; recessive only in pair. (iii)(A) Self-pollination of F₁; parental ratio 9 : 1; traits inherited independently. (iii)(B) Recessive trait hidden in F₁ reappears in F₂.

Where the marks are

All F₁ plants tall (½ mark)
No medium-height / blended plants (½ mark)
Correct difference between dominant and recessive trait (1 mark)
Self-pollination of F₁ (½ mark)
Parental combination ratio 9 : 1 (½ mark)
Conclusion: independent inheritance of traits (1 mark)
(iii)(B) Explanation with F₁ showing only dominant trait and F₂ reappearance of recessive trait (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 paper2024 board paper, set 31/1/1: every question, by section.

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CBSE Class 10 Science 2024: question 38

Hints

Step-by-step solution

Revise the chapter

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