Genetics and Evolution Revision Notes - Key Concepts & Important Questions

Genetics and Evolution is an important unit in Biology that explains how traits are inherited from one generation to the next and how species evolve over time. Key concepts like Mendelian inheritance, chromosomal theory of inheritance, molecular basis of inheritance, genetic variation, and the principles of evolution are covered in this unit. It also explains mechanisms like natural selection, mutation, genetic drift, and speciation that contribute to the diversity of life on Earth.

These revision notes on Genetics and Evolution, prepared by ALLEN’s expert faculty, provide a concise overview of the unit’s key concepts, important principles, and evolutionary mechanisms. Designed for quick and effective revision, they help students strengthen conceptual clarity and prepare efficiently for board and competitive examinations.

1.0Concept-Wise Important Revision Notes

1. Define genetics, heredity, variation and inheritance?

• Genetics: Branch of biology that studies how traits are passed from parents to offspring through genes.
• Heredity: Transmission of genetic characters from parents to progeny.
• Variation: Differences between offspring and their parents.
• Inheritance: The process through which characters are transmitted from parents to offspring.

2. Who is the father of genetics?

Father of genetics is Gregor J Mendel

3. What are character, trait and allele?

Character

• Characteristic feature of an organism.
• Example: Stem height.

Trait

• Variable form of a character.
• Example: Tall or dwarf.

Allele

• Alternate forms of a gene located at homologous sites on homologous chromosomes.

4. When did Mendel perform his experiments and on which organism? Also mention the reasons for his success.

Gregor Mendel performed experiments on garden pea (Pisum sativum) between 1856 and 1863.

Reasons for Mendel’s success include:

• Statistical and mathematical analysis of data
• Large sampling size
• Confirmation of inferences through experiments on successive generations

5. Why did Mendel’s work remain unrecognised for many years? Who rediscovered his work later?

Reasons include:

• Communication was not easy at that time
• Mathematical approach used by Mendel was new and unacceptable to many biologists
• He could not provide physical proof for the existence of factors
• Use of mathematics in biology

In 1900, Mendel’s work was rediscovered independently by:

• Carl Correns
• Hugo de Vries
• Erich von Tschermak

6. What characters did Mendel study in pea plants?

Mendel studied seven characters in pea plants with each character having two contrasting traits, including:

• Stem height
• Flower colour
• Flower position
• Pod shape
• Pod colour
• Seed shape
• Seed colour

7. What were the main steps in Mendel’s experimental technique?

Mendel’s experimental steps included:

1. Selection of pure plants
2. Hybridisation between pure plants involving
3. 1. Emasculation
   2. Bagging
   3. Tagging
   4. Dusting
3. Selfing of F₁ hybrids to obtain F₂ generation

8. What are monohybrid and dihybrid crosses?

• Monohybrid cross: Cross done to study inheritance of one character or one pair of contrasting traits.
• Dihybrid cross: Cross done to study inheritance of two characters or two pairs of contrasting traits.

9. What is the law of dominance?

The law of dominance states that:

• Characters are controlled by discrete units called factors
• Factors occur in pairs
• In a dissimilar pair, one member dominates the other

10. What is the law of segregation?

This law states that the two alleles of a gene separate from each other during gamete formation, so each gamete receives only one allele of a pair.

• The alleles do not blend with each other.
• Both characters are recovered in the F₂ generation, even though one may not appear in the F₁ generation.

11. What is the law of independent assortment?

The law states that: When two traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.

12. What is incomplete dominance?

Incomplete dominance occurs when:

• Phenotype of F₁ does not resemble either parent
• The phenotype is intermediate between the two parents
• Dominant allele is not completely dominant over the recessive allele

Examples include:

• Flower colour in snapdragon (Antirrhinum)
• Size of starch grain in pea plant

13. What is codominance?

In codominance:

• Both alleles are equally dominant in heterozygous condition
• F₁ resembles both parents

Example: AB blood group in humans

14. What are multiple alleles?

Multiple alleles are those that have more than two alternative forms of the same gene

Example: ABO blood group controlled by gene I

15. What is the chromosomal theory of inheritance?

In 1902, behaviour of chromosomes during meiosis was studied. Walter Sutton and Theodore Boveri noted that behaviour of chromosomes was parallel to behaviour of genes.

16. What are some genetic disorders?

Examples include:

• Thalassemia
• Down syndrome
• Klinefelter’s syndrome
• Turner’s syndrome

17. What are the theories for the origin of life?

Important theories include:

Theory of special creation

• Living organisms were created as such
• Diversity remained the same since creation
• Earth is about 4000 years old

Theory of panspermia

• Unit of life called spores transferred to planets including Earth

Theory of spontaneous generation

• Life originated from decaying matter such as straw or mud

Theory of biogenesis

• Life arises only from pre-existing life

Oparin–Haldane theory

• First life forms originated from non-living organic molecules

18. What evidence supports chemical evolution?

Evidence includes:

• S. L. Miller’s experiment (1953) that created primitive earth conditions
• Analysis of meteorite content showing similar organic compounds

19. What is the evidence of evolution?

Major evidences include:

• Paleontological evidence
• Comparative anatomy and morphology
• Biochemical similarities
• Embryological evidence

20. What does paleontological evidence show?

Study of fossils in sedimentary layers indicates:

• Geological period in which organisms existed
• Different rock layers contain fossils of different life forms
• Some fossils represent extinct organisms such as dinosaurs

21. What are Lamarck’s, Darwin’s and mutation theories of evolution?

Lamarck’s theory

• Inheritance of acquired characters
• Use and disuse of organs

Darwin’s theory

• Branching descent
• Natural selection

Mutation theory (Hugo de Vries)

• Evolution occurs due to mutations rather than minor variations

Also Read: Origin and Evolution of Life

22. What are the types of natural selection?

Natural selection can act in three ways:

• Directional selection: Favors individuals at one extreme of a trait distribution
• Stabilizing selection: Favors average or intermediate traits
• Disruptive selection: Favors both extremes of a trait but not the average

2.0Memory Tricks

Scientists Who Rediscovered Mendel’s Work (1900): “Correct Discoveries Today”

C → Carl Correns
D → Hugo de Vries
T → Erich Von Tschermak

3.0Steps in Mendel’s Experiment: “Select Hybrid Self”

S → Selection of pure plants
H → Hybridisation (Emasculation, Bagging, Tagging, Dusting)
S → Selfing of F1 to produce F2

4.0Genetic Disorders Mentioned: “Down Kids Try Therapy”

D → Down syndrome
K → Klinefelter syndrome
T → Turner syndrome
T → Thalassemia

5.0Theories of Origin of Life: “Some People Study Biology Often”

S → Special creation
P → Panspermia
S → Spontaneous generation
B → Biogenesis
O → Oparin–Haldane theory

6.0Previous Year Questions on Genetics and Evolution

Question: The ABO blood group system in humans is controlled by how many alleles?

A. Two
B. Three
C. Four
D. Five

Answer: Three

Question: Who proposed the Chromosomal Theory of Inheritance?

A. Mendel and Bateson
B. Watson and Crick
C. Sutton and Boveri
D. Darwin and Wallace

Answer: Sutton and Boveri

Question: In a dihybrid cross, the phenotypic ratio in the F₂ generation is:

A. 3 : 1
B. 1 : 2 : 1
C. 9 : 3 : 3 : 1
D. 1 : 1 : 1 : 1

Answer: 9 : 3 : 3 : 1

Question: DNA replication in the cell cycle occurs during:

A. G₁ phase
B. S phase
C. G₂ phase
D. M phase

Answer: S phase