Which of the following defines the term 'allele'?

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

• The physical expression of a gene.
• The entire genetic make-up of an organism.
• A segment of DNA that codes for a protein.
• Alternate forms of a gene.
Correct Option: D  [ Alternate forms of a gene ]

Remark: Alleles are alternate forms of a gene found at the same locus on homologous chromosomes, such as T (tall) and t (dwarf).

The progeny of a cross between two heterozygous parents (AaBb × AaBb) in a Mendelian dihybrid cross will show how many different phenotypes?

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

• Two
• Three
• Four
• Nine
Correct Option: C  [ Four ]

Remark: A dihybrid cross produces four phenotypic combinations: A_B_, A_bb, aaB_, and aabb.

Which cross involves crossing two individuals that are heterozygous for one trait (e.g., Rr × Rr)?

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

• Test cross
• Dihybrid cross
• Back cross
• Monohybrid cross
Correct Option: D  [ Monohybrid cross ]

Remark: A monohybrid cross involves tracking the inheritance of a single character, typically done by crossing two heterozygotes for that trait.

In the four o'clock plant, the cross of a pink flower with a white flower yields 50% pink and 50% white flowers. This result is characteristic of:

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

• Complete dominance
• Test cross for a dominant allele
• Incomplete dominance
• Reciprocal cross
Correct Option: C  [ Incomplete dominance ]

Remark: In incomplete dominance, Pink is heterozygous (RW) and White is homozygous recessive (WW or rr). The cross RW × WW yields 1/2 RW (Pink) and 1/2 WW (White), a 1:1 phenotypic ratio.

The 'factors' mentioned by Mendel that determine phenotype were later termed as:

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

• Genes
• Chromosomes
• Proteins
• Nucleic acids
Correct Option: A  [ Genes ]

Remark: Mendel's 'factors' are the units of inheritance, which were later identified as genes located on chromosomes.

If an individual is heterozygous for a trait (Aa), what percentage of their gametes will contain the recessive allele 'a'?

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

• 100%
• 75%
• 50%
• 25%
Correct Option: C  [ 50% ]

Remark: According to the Law of Segregation, the two alleles separate, so 50% of the gametes will carry 'A' and 50% will carry 'a'.

The phenomenon where the phenotype of the heterozygote is exactly the same as the homozygous dominant is known as:

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

• Incomplete dominance
• Co-dominance
• Complete dominance
• Pleiotropy
Correct Option: C  [ Complete dominance ]

Remark: Complete dominance is the standard Mendelian case where the dominant allele completely masks the recessive allele in the heterozygote.

When two pure lines (homozygous parents) are crossed, the first generation of offspring is called the:

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

• P generation
• F2 generation
• Filial generation
• F1 generation
Correct Option: D  [ F1 generation ]

Remark: F1 (First Filial) generation refers to the first generation of offspring produced from the parental (P) generation cross.

Which term describes the organism's entire genetic makeup?

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

• Phenotype
• Allele
• Genotype
• Genome
Correct Option: D  [ Genome ]

Remark: The genome is the complete set of genetic material (DNA) present in a cell or organism. The genotype refers to the specific alleles for a particular trait.

Down's syndrome is a genetic disorder caused by the trisomy of which specific chromosome?

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

• Chromosome 13
• Chromosome 18
• Chromosome 21
• X chromosome
Correct Option: C  [ Chromosome 21 ]

Remark: Down's syndrome (Trisomy 21) is caused by the presence of an extra copy of chromosome 21, resulting in a total chromosome count of 47.