If we cross pure-bred tall (dominant) pea plant with pure-bred dwarf (recessive) pea plant we will get pea plants of $F_1$ generation. If we now self-cross the pea plant of $F_1$ generation, then we obtain pea plants of $F_2$ generation.

(a) What do the plants of $F_1$ generation look like ?

(b) State the ratio of tall plants to dwarf plants in $F_2$ generation.

(c) State the type of plants not found in $F_1$ generation but appeared in $F_2$ generation, mentioning the reason for the same.

Q: If we cross pure-bred tall (dominant) pea plant with pure-bred dwarf (recessive) pea plant we will get pea plants of $F_1$ generation. If we now self-cross the pea plant of $F_1$ generation, then we obtain pea plants of $F_2$ generation. (a) What do the plants of $F_1$ generation look like ? (b) State the ratio of tall plants to dwarf plants in $F_2$ generation. (c) State the type of plants not found in $F_1$ generation but appeared in $F_2$ generation, mentioning the reason for the same.

(a) All the plants of the $F_1$ generation are **tall**. There are no medium-height or dwarf plants. (b) The ratio of tall plants to dwarf plants in the $F_2$ generation is **$3:1$**. (c) **Dwarf (short) plants** were not found in the $F_1$ generation but appeared in the $F_2$ generation. **Reason:** In the $F_1$ plants, both the tallness ($T$) and shortness ($t$) factors (alleles) were inherited from the parents, resulting in the heterozygous genotype $Tt$. Since tallness is a dominant trait, it masks the recessive shortness trait, making all $F_1$ plants look tall. However, when these $F_1$ plants undergo self-pollination, the alleles segregate during gamete formation. In the $F_2$ generation, one-quarter ($25\%$) of the offspring inherit the recessive allele '$t$' from both parents, resulting in the homozygous recessive genotype $tt$, which allows the dwarf trait to be expressed.

Heredity

Rules for the Inheritance of Traits – Mendel's Contributions

Official Answer

(a) All the plants of the $F_1$ generation are tall. There are no medium-height or dwarf plants.

(b) The ratio of tall plants to dwarf plants in the $F_2$ generation is $3:1$ .

Reason: In the $F_1$ plants, both the tallness ( $T$ ) and shortness ( $t$ ) factors (alleles) were inherited from the parents, resulting in the heterozygous genotype $Tt$ . Since tallness is a dominant trait, it masks the recessive shortness trait, making all $F_1$ plants look tall. However, when these $F_1$ plants undergo self-pollination, the alleles segregate during gamete formation. In the $F_2$ generation, one-quarter ( $25\%$ ) of the offspring inherit the recessive allele ' $t$ ' from both parents, resulting in the homozygous recessive genotype $tt$ , which allows the dwarf trait to be expressed.

tall3:1dwarfrecessivedominantsegregationhomozygousheterozygousTttt

Marking Scheme

1Part (a): Correctly identifying that all $F_1$ plants are tall. (1 mark)
2Part (b): Correctly stating the phenotypic ratio of $3:1$ (tall to dwarf) in the $F_2$ generation. (1 mark)
3Part (c): Identifying dwarf plants and explaining that the recessive trait is masked in $F_1$ ( $Tt$ ) but expressed in $F_2$ when homozygous ( $tt$ ). (1 mark)

Hint

Recall that a dominant trait requires only one copy of the allele to be expressed (e.g., $Tt$ is tall), whereas a recessive trait requires both copies to be identical (e.g., $tt$ is dwarf).

Quick Oral Answer

In the $F_1$ generation, all plants are tall because the allele for tallness ( $T$ ) is dominant over the allele for dwarfness ( $t$ ). The dwarf trait reappears in the $F_2$ generation in a $3:1$ ratio because the alleles segregate during gamete formation, allowing the homozygous recessive combination ( $tt$ ) to form.

Analysis & Explanation

This question tests the fundamental principles of Mendelian inheritance, specifically the Law of Dominance and the Law of Segregation. In a monohybrid cross involving contrasting characters (tall vs. dwarf), the $F_1$ generation only expresses the dominant trait because a single copy of the dominant allele ( $T$ ) is sufficient for expression. The recessive trait ( $t$ ) remains unexpressed but is preserved and segregates during gamete formation, reappearing in a $3:1$ ratio in the $F_2$ generation when homozygous recessive ( $tt$ ) individuals are formed.

Common Mistakes

1Writing the genotypic ratio ( $1:2:1$ ) instead of the phenotypic ratio ( $3:1$ ) for part (b).
2Thinking that the dwarf trait was completely lost or destroyed in the $F_1$ generation rather than simply being masked.

Interesting Facts

Mendel's choice of garden peas (Pisum sativum) was brilliant because they naturally self-pollinate, have easily distinguishable contrasting traits, and grow quickly, allowing him to observe multiple generations in a short time.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2012?

This question carries 3 marks in the CBSE Class 10 Science 2012 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Heredity" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Rules for the Inheritance of Traits – Mendel's Contributions" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2012 paper?

This is a Short Answer question from Section A in the CBSE Class 10 Science 2012 paper.