Chapter 5: Principles of Inheritance and Variation – Case-Based Questions with Answers

CBSE Class 12 Biology Case-Based Questions (NCERT): Principles of Inheritance and Variation

Course & Examination Details

Course: CBSE Class 12 Biology
Unit: Unit II – Genetics and Evolution
Chapter: Chapter 5 – Principles of Inheritance and Variation
Prescribed Textbook: NCERT Biology Class XII
Examination: CBSE Class 12 Board Examination
Question Type: Case-Based / Source-Based Questions

Section A: Mendel’s Experiments and Laws of Inheritance

Case 1

Gregor Mendel crossed tall and dwarf pea plants and observed all tall plants in the F₁ generation.
Q. Explain this observation.
Answer: All F₁ plants were tall because the allele for tallness is dominant over dwarfness. The recessive allele remained masked but was not lost.

Case 2

In the F₂ generation of a monohybrid cross, dwarf plants reappeared.
Q. Which law explains this result?
Answer: The Law of Segregation explains the reappearance of dwarf plants due to separation of alleles during gamete formation.

Case 3

A student observes that traits do not blend across generations.
Q. Which Mendelian principle supports this?
Answer: The Law of Segregation supports non-blending inheritance by ensuring alleles retain their identity across generations.

Case 4

Two different traits assort independently in a dihybrid cross.
Q. Name the law responsible and state its condition.
Answer: The Law of Independent Assortment applies when genes are on different chromosomes or far apart on the same chromosome.

Case 5

A dihybrid cross does not show a 9:3:3:1 ratio.
Q. Give one possible genetic reason.
Answer: Linkage between genes on the same chromosome prevents independent assortment, altering the Mendelian ratio.

Section B: Monohybrid and Dihybrid Crosses

Case 6

A plant breeder performs a test cross.
Q. What information does this cross provide?
Answer: A test cross determines whether an individual with a dominant phenotype is homozygous or heterozygous.

Case 7

In a monohybrid cross, both TT and Tt plants appear tall.
Q. Why is phenotype identical despite different genotypes?
Answer: Due to dominance, the dominant allele expresses itself in both homozygous and heterozygous conditions.

Case 8

Four different phenotypes appear in a dihybrid cross.
Q. What does this indicate about gamete formation?
Answer: It indicates formation of four different gamete types due to independent assortment of alleles.

Case 9

A Punnett square is used in a genetics problem.
Q. State its significance.
Answer: A Punnett square predicts possible genotypes and phenotypes of offspring in a genetic cross.

Case 10

Uniformity is observed in the F₁ generation.
Q. Which Mendelian law explains this?
Answer: The Law of Dominance explains uniform expression of dominant traits in the F₁ generation.

Section C: Deviations from Mendelian Inheritance

Case 11

Red and white flowers produce pink offspring.
Q. Identify the inheritance pattern.
Answer: This is an example of incomplete dominance where the heterozygote shows an intermediate phenotype.

Case 12

The F₂ generation shows a 1:2:1 phenotypic ratio.
Q. Why does this ratio occur?
Answer: Each genotype produces a distinct phenotype due to absence of complete dominance.

Case 13

An individual has AB blood group.
Q. Which genetic phenomenon explains this?
Answer: Codominance explains AB blood group, where both IA and IB alleles express equally.

Case 14

A gene has three alternative forms in a population.
Q. Name this phenomenon.
Answer: This phenomenon is called multiple alleles, as seen in the ABO blood group system.

Case 15

A single gene mutation affects multiple body traits.
Q. What is this phenomenon called?
Answer: This phenomenon is known as pleiotropy, where one gene influences multiple phenotypes.

Section D: Chromosomal Theory, Linkage and Recombination

Case 16

Genes show behavior similar to chromosomes during meiosis.
Q. Which theory explains this observation?
Answer: The chromosomal theory of inheritance explains gene behavior based on chromosome movement during meiosis.

Case 17

Certain genes are inherited together repeatedly.
Q. Identify the genetic cause.
Answer: This occurs due to linkage, where genes are located close together on the same chromosome.

Case 18

Crossing over produces new gene combinations.
Q. Name this process and its stage.
Answer: Recombination occurs during prophase I of meiosis due to crossing over.

Case 19

Low recombination frequency is observed between two genes.
Q. What does this indicate?
Answer: It indicates that the genes are located close together on the same chromosome.

Case 20

A geneticist prepares a gene map.
Q. Which parameter is used for mapping?
Answer: Recombination frequency is used to determine relative gene distances on chromosomes.

Section E: Sex Determination, Mutation and Genetic Disorders

Case 21

A child’s sex depends on the father.
Q. Explain this statement genetically.
Answer: The father produces X- and Y-bearing sperms, determining whether the offspring is male or female.

Case 22

Females are heterogametic in birds.
Q. Name this sex determination system.
Answer: Birds follow the ZW–ZZ sex determination system, where females are ZW.

Case 23

A sudden heritable DNA change is observed.
Q. What is this change called?
Answer: Such a heritable change is called a mutation.

Case 24

A child is born with trisomy 21.
Q. Name the disorder and its cause.
Answer: The disorder is Down’s syndrome, caused by nondisjunction leading to an extra chromosome 21.

Case 25

A family has a history of haemophilia.
Q. What type of inheritance does this disorder follow?
Answer: Haemophilia follows X-linked recessive inheritance, affecting males more frequently.