A blue colour flower plant denoted by BB is crossbred with that of white colour flower plant denoted by bb. (a) State the colour of flower you would expect in their F1 generation plants. (b) What must be the percentage of white flower plants in F2 generation if flowers of F1 plants are self-pollinated ? (c) State the expected ratio of the genotypes BB and Bb in the F2 progeny.

Q: A blue colour flower plant denoted by BB is crossbred with that of white colour flower plant denoted by bb. (a) State the colour of flower you would expect in their F1 generation plants. (b) What must be the percentage of white flower plants in F2 generation if flowers of F1 plants are self-pollinated ? (c) State the expected ratio of the genotypes BB and Bb in the F2 progeny.

(a) **Colour of F1 generation flowers:** All plants in the $F_1$ generation will have **blue** flowers. This is because the allele for blue colour ($B$) is dominant over the allele for white colour ($b$), resulting in the heterozygous genotype $Bb$. (b) **Percentage of white flower plants in F2 generation:** When $F_1$ plants ($Bb$) are self-pollinated ($Bb \times Bb$), the $F_2$ generation will have a phenotypic ratio of 3 blue to 1 white. Therefore, the percentage of white flower plants ($bb$) will be **25%** (or $\frac{1}{4} \times 100\%$). (c) **Expected ratio of genotypes BB and Bb in F2 progeny:** The genotypic ratio in the $F_2$ generation is $1\text{ BB} : 2\text{ Bb} : 1\text{ bb}$. Thus, the ratio of the genotypes $BB$ to $Bb$ is **1:2**.

Heredity

Rules for the Inheritance of Traits – Mendel's Contributions

Official Answer

(a) Colour of F1 generation flowers: All plants in the $F_1$ generation will have blue flowers. This is because the allele for blue colour ( $B$ ) is dominant over the allele for white colour ( $b$ ), resulting in the heterozygous genotype $Bb$ .

(b) Percentage of white flower plants in F2 generation: When $F_1$ plants ( $Bb$ ) are self-pollinated ( $Bb \times Bb$ ), the $F_2$ generation will have a phenotypic ratio of 3 blue to 1 white. Therefore, the percentage of white flower plants ( $bb$ ) will be 25% (or $\frac{1}{4} \times 100\%$ ).

(c) Expected ratio of genotypes BB and Bb in F2 progeny: The genotypic ratio in the $F_2$ generation is $1\text{ BB} : 2\text{ Bb} : 1\text{ bb}$ . Thus, the ratio of the genotypes $BB$ to $Bb$ is 1:2.

blue25%1:2dominantrecessivegenotypephenotypePunnett square

Marking Scheme

1Stating blue colour for F1 generation with correct genotype explanation. (1 mark)
2Calculating 25% white flowers in F2 generation with Punnett square or ratio. (1 mark)
3Stating the ratio of BB to Bb as 1:2. (1 mark)

Hint

Perform a monohybrid cross similar to Mendel's tall ( $TT$ ) and short ( $tt$ ) pea plants, replacing $T$ with $B$ and $t$ with $b$ .

Quick Oral Answer

In the F1 generation, all flowers are blue because blue is dominant. In the F2 generation, 25% are white, and the ratio of BB to Bb genotypes is 1:2.

Analysis & Explanation

This question is a direct application of Mendel's monohybrid cross. The allele $B$ represents the dominant trait (blue flower) and $b$ represents the recessive trait (white flower). In the $F_1$ generation, all offspring inherit one allele from each parent, resulting in the heterozygous genotype $Bb$ . Since $B$ is dominant, the phenotype is blue. In the $F_2$ generation, self-pollination of $Bb$ yields genotypes $BB$ , $Bb$ , and $bb$ in a $1:2:1$ ratio. The recessive phenotype (white, $bb$ ) only appears when both copies are recessive, which constitutes $25\%$ of the offspring. The ratio of homozygous dominant ( $BB$ ) to heterozygous ( $Bb$ ) is $1:2$ .

Common Mistakes

1Writing the ratio of BB to Bb as 1:3 or 3:1 by confusing phenotypic and genotypic ratios.
2Assuming F1 generation flowers will be a blend (like light blue) instead of the dominant blue.

Interesting Facts

Mendel's choice of discrete contrasting characters (like violet vs. white flowers) was crucial because it avoided the blending inheritance theory popular at the 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.