If a heterozygous tall pea plant (Tt) is self-pollinated, what will be the ratio of tall to dwarf plants in the offspring?

[Principles-of-Inheritance-and-Variation] [class-xii ]

• 1 : 1
• 2 : 1
• 3:1
• 1:3
Correct Option: C  [ 3:1 ]

Remark:
Answer: (c) 3 : 1

Explanation: When a heterozygous tall pea plant (Tt) is self-pollinated, the genotypes formed are TT, Tt, and tt in the ratio 1 : 2 : 1. Since both TT and Tt produce tall plants, the phenotypic ratio of tall to dwarf is 3 : 1.

When a red-flowered snapdragon (RR) is crossed with a white-flowered snapdragon (rr), the F₁ generation has pink flowers. This is an example of:

[Principles-of-Inheritance-and-Variation] [class-xii ]

• Codominance
• Incomplete dominance
• Multiple allelism
• Epistasis
Correct Option: B  [ Incomplete dominance ]

Remark:
Answer: (b) Incomplete dominance

Explanation: In incomplete dominance, neither allele is completely dominant over the other. In snapdragon (Antirrhinum majus), crossing red (RR) and white (rr) results in pink (Rr) flowers, showing an intermediate phenotype.

Haemophilia is an example of a:

[Principles-of-Inheritance-and-Variation] [class-xii ]

• Autosomal dominant disorder
• Autosomal recessive disorder
• Sex-linked dominant disorder
• Sex-linked recessive disorder
Correct Option: D  [ Sex-linked recessive disorder ]

Remark:
Answer: (d) Sex-linked recessive disorder

Explanation: Haemophilia is caused by a mutation in a gene present on the X chromosome. Since males have only one X chromosome (XY), the presence of the defective gene causes the disease. Females (XX) are usually carriers because a normal allele on the other X can compensate. The disorder leads to delayed clotting of blood due to deficiency of clotting factors.

A mutation that arises due to a change in a single base pair of DNA is known as a:

[Principles-of-Inheritance-and-Variation] [class-xii ]

• Chromosomal aberration
• Point mutation
• Frameshift mutation
• Polyploidy
Correct Option: B  [ Point mutation ]

Remark: A change in a single base pair of DNA is known as a point mutation. A classical example is sickle cell anemia.

Physical or chemical factors that induce mutations are referred to as:

[Principles-of-Inheritance-and-Variation] [class-xii ]

• Alleles
• Carcinogens
• Mutagens
• Teratogens
Correct Option: C  [ Mutagens ]

Remark: There are many chemical and physical factors that induce mutations, and these are referred to as mutagens. UV radiation is given as an example of a mutagen.

The analysis of traits in several generations of a family using a family tree is called:

[Principles-of-Inheritance-and-Variation] [class-xii ]

• Karyotyping
• A test cross
• Pedigree analysis
• Amniocentesis
Correct Option: C  [ Pedigree analysis ]

Remark: The study of family history about the inheritance of a particular trait is called pedigree analysis, where the inheritance of a trait is represented in the family tree over generations.

Genetic disorders that are mainly determined by alteration or mutation in a single gene are called:

[Principles-of-Inheritance-and-Variation] [class-xii ]

• Chromosomal disorders
• Mendelian disorders
• Polygenic disorders
• Infectious diseases
Correct Option: B  [ Mendelian disorders ]

Remark: Mendelian disorders are mainly determined by alteration or mutation in a single gene. Their inheritance pattern can be traced using pedigree analysis.

Colour blindness is a:

[Principles-of-Inheritance-and-Variation] [class-xii ]

• Sex-linked recessive disorder
• Autosomal dominant disorder
• Sex-linked dominant disorder
• Autosomal recessive disorder
Correct Option: A  [ Sex-linked recessive disorder ]

Remark: Colour blindness is a sex-linked recessive disorder caused by a mutation in certain genes present on the X chromosome.

Which of the following disorders is a sex-linked recessive disease where a single protein involved in blood clotting is affected?

[Principles-of-Inheritance-and-Variation] [class-xii ]

• Sickle-cell anaemia
• Phenylketonuria
• Haemophilia
• Thalassemia
Correct Option: C  [ Haemophilia ]

Remark: Haemophilia is a sex linked recessive disease in which a single protein that is part of the cascade of proteins involved in the clotting of blood is affected, resulting in non-stop bleeding from a simple cut.

Sickle-cell anaemia is an:

[Principles-of-Inheritance-and-Variation] [class-xii ]

• Autosome linked recessive trait
• X-linked recessive trait
• Autosome-linked dominant trait
• X-linked dominant trait
Correct Option: A  [ Autosome linked recessive trait ]

Remark: Sickle-cell anaemia is an autosome linked recessive trait that can be transmitted from parents to the offspring when both partners are carriers for the gene.