Genes which code for a pair of contrasting traits are known as:

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

• Homologues
• Genotypes
• Phenotypes
• Alleles
Correct Option: D  [ Alleles ]

Remark: Alleles are slightly different forms of the same gene. They are genes that code for a pair of contrasting traits. For example, T (for tall) and t (for dwarf) are alleles of each other.

If an organism has two identical alleles for a particular gene (e.g., TT or tt), its genotype is described as:

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

• Heterozygous
• Dominant
• Recessive
• Homozygous
Correct Option: D  [ Homozygous ]

Remark: Mendel proposed that in a true-breeding variety, the allelic pair of genes for a trait are identical, or homozygous. TT and tt are examples of homozygous genotypes.

A pea plant with the genotype Tt is best described as:

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

• Homozygous dominant
• Homozygous recessive
• A monohybrid or heterozygous
• A dihybrid
Correct Option: C  [ A monohybrid or heterozygous ]

Remark: Since the Tt plant is heterozygous for genes controlling one character (height), it is a monohybrid. Plants with dissimilar alleles (Tt) are heterozygous.

Who developed the graphical representation used to calculate the probability of all possible genotypes of offspring in a genetic cross?

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

• Gregor Mendel
• Walter Sutton
• T.H. Morgan
• Reginald C. Punnett
Correct Option: D  [ Reginald C. Punnett ]

Remark: The diagram used to understand the formation of zygotes and F1 and F2 plants is called a Punnett Square. It was developed by a British geneticist, Reginald C. Punnett.

In a Mendelian monohybrid cross, what is the genotypic ratio in the F2 generation?

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

• 3:1
• 1:1
• 1:2:1
• 9:3:3:1
Correct Option: C  [ 1:2:1 ]

Remark: While the F2 phenotypic ratio is 3:1, the genotypic ratio is 1/4 (TT) : 1/2 (Tt) : 1/4 (tt), which simplifies to 1:2:1.

A cross between an organism with a dominant phenotype (of unknown genotype) and its recessive parent is called a:

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

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

Remark: To determine the genotype of a tall plant at F2, Mendel crossed the tall plant with a dwarf plant. This is called a test cross, where an organism with a dominant phenotype is crossed with a recessive parent.

Which of the following statements is part of Mendel's Law of Dominance?

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

• Characters are controlled by discrete units called factors.
• Factors occur in pairs.
• In a dissimilar pair of factors, one member dominates the other.
• All of the above.
Correct Option: D  [ All of the above. ]

Remark: The Law of Dominance states that: (i) Characters are controlled by discrete units called factors; (ii) Factors occur in pairs; (iii) In a dissimilar pair of factors, one member dominates the other.

The Law of Segregation is based on the fact that:

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

• Alleles show blending in the F1 generation.
• Alleles do not show any blending and both characters are recovered in the F2 generation.
• Only dominant characters are passed on to the next generation.
• Factors for different traits are linked.
Correct Option: B  [ Alleles do not show any blending and both characters are recovered in the F2 generation. ]

Remark: The Law of Segregation is based on the fact that the alleles do not show any blending and that both characters are recovered as such in the F2 generation, though one is not seen at the F1 stage.

In the dog flower (Antirrhinum sp.), a cross between true-breeding red-flowered (RR) and true-breeding white-flowered (rr) plants results in F1 progeny that are all pink. This is an example of:

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

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

Remark: In the dog flower, the F1 (Rr) from a cross between red (RR) and white (rr) flowered plants was pink. This phenotype is in between the two parents and is a good example of incomplete dominance.

What is the phenotypic ratio in the F2 generation of a cross showing incomplete dominance, like the flower colour in snapdragons?

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

• 3:1
• 1:1
• 9:3:3:1
• 1:2:1
Correct Option: D  [ 1:2:1 ]

Remark: When the pink F1 (Rr) was self-pollinated, the F2 resulted in a ratio of 1 (RR) Red : 2 (Rr) Pink : 1 (rr) White. Here the phenotypic ratio is 1:2:1.