Questions
3-5 questions per paper
Difficulty
Medium
Importance
Core — never skip
Overview
Heredity and Evolution explore the transmission of traits from parents to offspring and the gradual changes in biological populations over time. Mastery of this topic is essential for CBSE boards as it bridges genetics and natural selection, forming the backbone of modern biological sciences.
Mendelian Genetics
Gregor Mendel's experiments on pea plants established the fundamental laws of inheritance through monohybrid and dihybrid crosses. Understanding the F1 and F2 generation ratios is crucial for solving numerical problems regarding dominant and recessive alleles.
- Law of Segregation: Alleles separate during gamete formation
- Law of Independent Assortment: Different traits are inherited independently
- Monohybrid ratio: 3:1 phenotypic, 1:2:1 genotypic
- Dihybrid phenotypic ratio: 9:3:3:1
- Allele: Variant form of a given gene
Sex Determination
Human sex determination is genetically controlled by the presence of X and Y chromosomes inherited from parents. Since the male produces two types of gametes, he is the primary determinant of the biological sex of the offspring.
- Human karyotype: 46 chromosomes (23 pairs)
- Male genotype: XY (heterogametic)
- Female genotype: XX (homogametic)
- Probability of male or female birth is always 50%
Evolution and Speciation
Evolution involves the accumulation of variations over generations leading to new species, driven primarily by natural selection and genetic drift. It is important to distinguish between acquired and inherited traits to understand evolutionary mechanisms.
- Darwin’s theory: Natural selection (survival of the fittest)
- Genetic drift: Random changes in allele frequency
- Speciation: Formation of new species through geographical isolation
- Homologous organs: Common ancestry, different functions
- Analogous organs: Different origin, similar functions
Exam Tip
When solving genetic cross problems, always draw a Punnett Square to visualize gamete combinations and avoid simple arithmetic errors.
Common Mistakes
- Confusing acquired traits (e.g., muscle building) with inherited traits that appear in germ cells.
- Miscalculating the 9:3:3:1 dihybrid ratio by failing to assign parental gamete combinations correctly.
- Assuming evolution is synonymous with the progression of a simple organism into a complex one.
More Revision Notes
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