Cheat sheet · Class 12 · CBSE · Science Biology
Principles of Inheritance and Variation cheat sheet
The whole chapter on one page: key formulas, facts and the points to remember, for quick revision before a test.
CBSE Class 12 · Biology
Cheat sheet
Chapter 3Principles of Inheritance and Variation
Mendel's ratios: monohybrid 3 : 1, dihybrid 9 : 3 : 3 : 1, test cross 1 : 1.
1. Mendel's Laws
- Law of dominance.
- Law of segregation (purity of gametes).
- Law of independent assortment (dihybrid cross).
2. Deviations
| Pattern | Example | F₂ ratio |
|---|---|---|
| Incomplete dominance | snapdragon flower colour | 1 : 2 : 1 |
| Co-dominance | ABO blood group (Iᴬ Iᴮ) | 1 : 2 : 1 |
| Multiple alleles | ABO (3 alleles) | — |
| Pleiotropy | phenylketonuria | — |
| Polygenic | human skin colour | continuous |
3. Chromosomal Theory
- Sutton and Boveri.
- Morgan (Drosophila): linkage and recombination.
- Sex determination: XX–XY (humans), XX–XO (grasshopper), ZW–ZZ (birds).
4. Disorders
| Disorder | Type |
|---|---|
| Haemophilia, colour blindness | X-linked recessive |
| Sickle-cell anaemia | autosomal recessive (Glu → Val) |
| Thalassaemia | autosomal recessive |
| Down's syndrome | trisomy 21 |
| Klinefelter's | XXY |
| Turner's | XO |
5. Key Points
- Test cross reveals genotype.
- Linked genes are inherited together.
- Pedigree analysis traces traits.
- Mutations: point, frameshift.
Remember: 3 laws | deviations | sex determination | disorders
Principles of Inheritance and Variation: questions and notes
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