Assertion (A) : In humans, if gene (B) is responsible for black eyes and gene (b) is responsible for brown eyes, then the colour of eyes of the progeny having gene combination Bb, bb or BB will be black only.

Reason (R) : The black colour of the eyes is a dominant trait.

Heredity

Rules for the Inheritance of Traits – Mendel's Contributions

Options

(a)Both (A) and (R) are true and (R) is the correct explanation of (A).

(b)Both (A) and (R) are true but (R) is not the correct explanation of (A).

(c)(A) is true but (R) is false.

(d)(A) is false but (R) is true.

Official Answer

The correct option is (d). The Assertion (A) is false because individuals with the gene combination $bb$ will have brown eyes, not black. The Reason (R) is true because the gene for black eyes ( $B$ ) is dominant over the gene for brown eyes ( $b$ ).

dominant traitrecessive traithomozygous recessiveBbbbBB

Marking Scheme

1Stating that Assertion is false because genotype $bb$ results in brown eyes: 0.5 marks
2Stating that Reason is true because black eye colour ( $B$ ) is dominant over brown ( $b$ ): 0.5 marks

Hint

Identify the phenotype of the homozygous recessive genotype $bb$ . If $b$ represents brown eyes, will a person with $bb$ have black eyes?

Quick Oral Answer

A dominant allele like $B$ masks the expression of a recessive allele like $b$ . Therefore, $BB$ and $Bb$ show the dominant phenotype (black eyes), while $bb$ shows the recessive phenotype (brown eyes).

Analysis & Explanation

According to the rules of inheritance, a dominant trait is expressed even when only a single copy of the dominant allele is present (e.g., $BB$ and $Bb$ both result in black eyes). A recessive trait is only expressed when both copies of the allele are recessive (e.g., $bb$ results in brown eyes).

Assertion (A) is false: It states that the eye colour of progeny with genotypes $Bb$ , $bb$ , or $BB$ will be black only. However, the genotype $bb$ consists of two recessive alleles, which will express the recessive trait, resulting in brown eyes.
Reason (R) is true: The black eye colour trait (represented by $B$ ) is dominant over the brown eye colour trait (represented by $b$ ). This explains why $BB$ and $Bb$ genotypes result in black eyes.

Since Assertion is false and Reason is true, option (d) is the correct choice.

Common Mistakes

1Students might assume that since $B$ is dominant, all combinations containing $b$ will also be black, forgetting that the homozygous recessive genotype $bb$ will express the recessive brown eye trait.

Interesting Facts

In humans, eye colour is actually a polygenic trait controlled by multiple genes, but simplified Mendelian models (like $B$ and $b$ ) are often used to teach basic inheritance patterns.

Spotted a mistake or something unclear?

Tell us — we fix reported answers fast.

Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2023?

This question carries 1 mark in the CBSE Class 10 Science 2023 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 2023 paper?

This is a MCQ question from Section A in the CBSE Class 10 Science 2023 paper.