Chapter 5 — Principles of Inheritance and Variation — is the genetics chapter, one of the most important in Class 12 Biology with 10-12 marks in Board exams. It covers Mendel’s laws, chromosomal theory, linkage, sex determination, and genetic disorders. Master the crosses and pedigree analysis!
Key Concepts
Mendel’s Laws of Inheritance
Law of Segregation (Law of Purity of Gametes): During gamete formation, the two alleles of a gene segregate so each gamete receives only one allele.
Law of Independent Assortment: Alleles of different genes assort independently during gamete formation (applies when genes are on different chromosomes).
Monohybrid Cross (Tt × Tt)
F2 Phenotype ratio: 3 Tall : 1 Dwarf
Test cross (Tt × tt): 1 Tall : 1 Dwarf (confirms heterozygosity)
Dihybrid Cross (RrYy × RrYy)
9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green
Incomplete Dominance
Neither allele is completely dominant. F1 shows intermediate phenotype.
RR (Red) × rr (White) → Rr (Pink) in F1
F2 ratio: 1 Red : 2 Pink : 1 White (1:2:1)
Codominance
Both alleles express themselves simultaneously.
I^A I^A or I^A i → Blood group A
I^B I^B or I^B i → Blood group B
I^A I^B → Blood group AB (codominance — both A and B antigens present)
ii → Blood group O
Sex Determination
| System | Male | Female | Examples |
|---|---|---|---|
| XX-XY | XY (heterogametic) | XX (homogametic) | Humans, Drosophila |
| ZZ-ZW | ZZ (homogametic) | ZW (heterogametic) | Birds, some reptiles |
| XX-XO | XO | XX | Grasshoppers |
Sex-Linked Inheritance
X^c Y (affected male) × X^C X^C (normal female) → all normal (carrier daughters)
X^C X^c (carrier female) × X^C Y (normal male) → 50% sons affected
Haemophilia (X-linked recessive): Same pattern as colour blindness
Males are more commonly affected (only need one defective allele on X)
Linkage and Recombination
- Linkage: Genes on the same chromosome tend to be inherited together (discovered by Morgan using Drosophila)
- Recombination: Crossing over breaks linkage → new combinations of genes
- Recombination frequency = (recombinant offspring / total offspring) × 100
- Tightly linked genes → low recombination frequency
Chromosomal Disorders
| Disorder | Chromosomal Condition | Features |
|---|---|---|
| Down Syndrome | Trisomy 21 (47 chromosomes) | Mental retardation, short stature, broad face |
| Turner Syndrome | 45, XO (monosomy) | Female, short stature, sterile, webbed neck |
| Klinefelter Syndrome | 47, XXY (trisomy) | Male, gynaecomastia, sterile |
Mendelian Disorders
| Disorder | Inheritance | Feature |
|---|---|---|
| Sickle Cell Anaemia | Autosomal recessive | HbS (Glu→Val at position 6 of β-chain) |
| Phenylketonuria | Autosomal recessive | Cannot convert phenylalanine → tyrosine |
| Thalassemia | Autosomal recessive | Reduced synthesis of α or β chains of haemoglobin |
| Colour Blindness | X-linked recessive | Cannot distinguish red-green colours |
| Haemophilia | X-linked recessive | Blood doesn’t clot properly |
Solved Examples
Example 1: Monohybrid Cross
Q: In pea plants, tall (T) is dominant over dwarf (t). Cross a heterozygous tall plant with a dwarf plant. What is the expected phenotype ratio?
Solution: Tt × tt → Tt, Tt, tt, tt → 1 Tall : 1 Dwarf (test cross ratio)
Example 2: Blood Groups
Q: A mother has blood group A (I^A i) and father has blood group B (I^B i). What blood groups are possible in their children?
Solution: I^A i × I^B i → I^A I^B (AB), I^A i (A), I^B i (B), ii (O). All four blood groups are possible!
Important Questions for Board Exams
1 Mark
- State the law of segregation.
- What is a test cross?
- Why is haemophilia more common in males?
3 Mark
- Explain codominance with ABO blood group example.
- Describe the chromosomal basis of sex determination in humans.
- What is Down syndrome? What is its chromosomal basis?
5 Mark
- Describe Mendel’s dihybrid cross. State the law of independent assortment. Give the F2 phenotypic and genotypic ratios.
- What is sex-linked inheritance? Explain the inheritance of colour blindness with a cross.
Quick Revision Points
- Mendel: 3 laws — dominance, segregation, independent assortment
- Monohybrid F2: 3:1; Dihybrid F2: 9:3:3:1; Test cross: 1:1
- Incomplete dominance: 1:2:1 (snapdragon); Codominance: ABO blood groups
- Sex determination: XX-XY (humans); ZZ-ZW (birds)
- Colour blindness & haemophilia: X-linked recessive → males more affected
- Down: trisomy 21; Turner: 45,XO; Klinefelter: 47,XXY
- Sickle cell: autosomal recessive, Glu→Val in β-globin
- Linkage: Morgan, Drosophila; Recombination frequency used for gene mapping
Chapter Navigation
Previous: Reproductive Health Class 12 Notes
Next: Molecular Basis of Inheritance Class 12 Notes
Related Chapters in Class 12 Biology
- Molecular Basis of Inheritance Class 12 Notes
- Evolution Class 12 Notes
- Reproductive Health Class 12 Notes
Practice What You Learned
Test yourself with our NEET Mock Test Set 1 to see how well you’ve mastered the concepts.