Principles of Inheritance and Variation

NCERT Line-by-Line Breakdown for NEET 2026

Unit 2: Genetics and Evolution

Genetics is the branch of biology dealing with Inheritance (process by which characters are passed from parent to progeny) and Variation (degree by which progeny differ from their parents). The father of genetics is Gregor Johann Mendel.

1. Mendel’s Laws of Inheritance

Mendel conducted hybridization experiments on garden peas (Pisum sativum) for 7 years (1856-1863). He selected 14 true-breeding pea plant varieties (7 pairs of contrasting traits).

[Image of Mendel’s seven characters of pea plant]

A. Inheritance of One Gene (Monohybrid Cross)

Cross between two parents differing in one pair of contrasting characters (e.g., Tall vs Dwarf).

F1 Generation: All Tall. (Resembled only one parent).
F2 Generation: 3 Tall : 1 Dwarf (Phenotypic ratio). 1 TT : 2 Tt : 1 tt (Genotypic ratio).

📜 Mendel’s Postulates:

Law of Dominance: Characters are controlled by discrete units called Factors (Genes). Factors occur in pairs. In a dissimilar pair, one dominates the other.
Law of Segregation: Alleles do not show any blending. During gamete formation, the paired alleles separate (segregate) so that a gamete receives only one of the two factors. (Universally accepted law with no exception).

B. Test Cross

Cross between an F1 hybrid (dominant phenotype) and its recessive parent. Used to determine the genotype of the dominant parent.
Ratio is always 1:1.

2. Deviations from Mendelism

A. Incomplete Dominance

F1 phenotype does not resemble either of the two parents and is in between the two.

Example: Flower color in Dog flower (Snapdragon / Antirrhinum sp.).
Cross: Red (RR) x White (rr) → Pink (Rr).
Phenotypic & Genotypic Ratio (F2): 1:2:1 (Red:Pink:White).

B. Co-dominance

F1 phenotype resembles both parents.

Example: ABO blood grouping in humans.
Gene I has 3 alleles: I^A, I^B, i.
I^A and I^B are co-dominant (produce both antigens). Blood group AB.

3. Chromosomal Theory & Linkage

Sutton and Boveri (1902): Noted that the behavior of chromosomes was parallel to the behavior of genes. Proposed the Chromosomal Theory of Inheritance.

Morgan’s Experiment (Fruit Fly)

Thomas Hunt Morgan worked with Drosophila melanogaster.

Linkage: Physical association of genes on a chromosome.
Recombination: Generation of non-parental gene combinations.
Conclusion: Tightly linked genes show very low recombination. Loosely linked genes show higher recombination.

4. Sex Determination & Mutation

Type	Organisms	Mechanism
XX-XY	Humans, Drosophila	Males Heterogametic (XY), Females Homogametic (XX).
XX-XO	Grasshopper	Males have only one X chromosome (XO).
ZZ-ZW	Birds	Females Heterogametic (ZW), Males Homogametic (ZZ).

Haplodiploidy: Found in Honeybees. Females are diploid (32), Males (Drones) are haploid (16) produced by parthenogenesis.

Mutation

Point Mutation: Change in single base pair (e.g., Sickle Cell Anemia).
Frame-shift Mutation: Deletion or insertion of base pairs.

5. Genetic Disorders

A. Mendelian Disorders (Gene related)

Haemophilia
Sex-linked recessive. “Bleeder’s disease”. Blood fails to clot. Queen Victoria was a carrier.

Sickle Cell Anemia
Autosomal recessive. Glutamic acid (Glu) replaced by Valine (Val) at 6th position of Beta-globin chain. RBCs become sickle-shaped.

Phenylketonuria (PKU)
Autosomal recessive. Lacks enzyme converting Phenylalanine to Tyrosine. Causes mental retardation.

B. Chromosomal Disorders

Down’s Syndrome: Trisomy of Chromosome 21. Short stature, furrowed tongue, mental retardation.
Klinefelter’s Syndrome: XXY (47). Male with feminine development (Gynecomastia). Sterile.
Turner’s Syndrome: XO (45). Sterile females. Rudimentary ovaries.

📝 Rapid Fire MCQs

Q1. Which of the following is an example of pleiotropy?

A) Haemophilia
B) Thalassemia
C) Sickle cell anemia
D) Colour blindness

Click to check Answer

Answer: C) Sickle cell anemia (One gene affects multiple traits).

Q2. In a test cross involving F1 dihybrid flies, more parental-type offspring were produced than the recombinant-type offspring. This indicates:

A) The two genes are linked and present on the same chromosome
B) Both of the characters are controlled by more than one gene
C) The two genes are located on two different chromosomes
D) Chromosomes failed to separate during meiosis