📌 Key Points
- Mendel's Law of Segregation: Alleles separate during meiosis, each gamete gets one allele
- Mendel's Law of Independent Assortment: Different traits segregate independently during meiosis
- Dominant allele expressed in heterozygotes (Aa shows dominant phenotype), recessive masked
- Monohybrid cross Aa × Aa: 1 AA : 2 Aa : 1 aa (genotype), 3 dominant : 1 recessive (phenotype)
- Dihybrid cross AaBb × AaBb: 9:3:3:1 phenotypic ratio demonstrating both laws
- DNA double helix with sugar-phosphate backbone and nitrogenous bases A, T, G, C
- Base pairing: A-T and G-C (complementary). Ensures accurate replication and inheritance
- Semi-conservative replication: Each new DNA has one old and one new strand
- Gene: Stretch of DNA coding for protein/trait. Allele: Variant form of gene
- Mutation: Change in DNA sequence (point, insertion, deletion, inversion). Creates variation
- Autosomal traits: Affect males and females equally. Autosomal dominant/recessive
- Sex-linked (X-linked): Gene on X chromosome. Males affected with one recessive allele, females need two
- Carrier: Heterozygous individual (Aa or AaX for X-linked) showing dominant phenotype but carrying recessive
- Pedigree analysis: Traces inheritance through families. Squares=male, circles=female, filled=affected
- Genetic variation from mutations and sexual reproduction enables evolution and adaptation
- Genotype: Genetic makeup (AA, Aa, aa). Phenotype: Observable trait
- Homozygous: Same alleles (AA or aa). Heterozygous: Different alleles (Aa)
- 3:1 ratio in F2 generation monohybrid cross shows Mendelian inheritance
- X-linked recessive traits (colorblindness, hemophilia) rare in females, common in males
- DNA → mRNA → Protein (genetic code) determines traits. Mutations alter protein production
📘 Important Definitions
⚠️ Common Mistakes
✗ Wrong: Dominant allele is stronger or more important.
✓ Correct: Dominance just means expressed phenotypically. Both alleles equally valid. Dominance is expression pattern, not quality.
✗ Wrong: Recessive traits skip generations so next generation won't have it.
✓ Correct: Recessive traits skip visible expression but carriers pass it. Can reappear when two carriers mate.
✗ Wrong: All mutations are harmful.
✓ Correct: Mutations can be beneficial, neutral, or harmful. Beneficial mutations increase survival (natural selection).
✗ Wrong: Sex-linked traits equally common in males and females.
✓ Correct: X-linked recessive rare in females (need 2 copies) but common in males (1 copy). 20x more common in males.
✗ Wrong: Punnett square shows actual offspring, not probability.
✓ Correct: Punnett square shows probability ratios. Each cross independent - actual results may vary.
✗ Wrong: DNA replication produces two identical strands from template.
✓ Correct: Semi-conservative: Each new DNA has one old strand (template) and one new strand. Both accurate copies.
📝 Exam Focus
These questions are frequently asked in CBSE exams:
🎯 Last-Minute Recall
Close your eyes and try to recall: Key definitions, formulas, and 3 common mistakes. If you can recall 80% without looking, you're exam-ready!