8
CBSE Marks
★★★★★
Difficulty
10
Topics
High
Board Weight
Topics Covered
10 key topics in this chapter
Accumulation of Variation
Heredity & Genetics
Mendel's Experiments
Rules of Inheritance
Dominant & Recessive Traits
Monohybrid & Dihybrid Cross
Sex Determination
Acquired vs Inherited Traits
Evolution: Natural Selection
Speciation & Evolutionary Relationships
Study Resources
Key Formulas & Reactions
| Formula / Reaction / Rule | Expression |
|---|---|
| Monohybrid Phenotype Ratio | \(3 : 1 (Dominant : Recessive)\) |
| Monohybrid Genotype Ratio | \(1 TT : 2 Tt : 1 tt\) |
| Dihybrid Phenotype Ratio | \(9 : 3 : 3 : 1\) |
| Sex Chromosomes | \(Female = XX, Male = XY\) |
| Acquired traits | \(NOT inherited — e.g. muscle gained by exercise\) |
| Inherited traits | \(Encoded in DNA — e.g. eye colour, blood group\) |
Important Points to Remember
Mendel's Law of Segregation: alleles separate during gamete formation — each gamete carries only one allele per gene.
Mendel's Law of Independent Assortment: genes for different traits segregate independently (if on different chromosomes).
Monohybrid F₂ ratio: 3 Dominant : 1 Recessive (phenotype). Genotypic: 1 TT : 2 Tt : 1 tt.
Sex determination in humans: XX = female, XY = male. It is the father's sperm (X or Y) that determines the sex of the child.