Mendelian Genetics

• Mendel's laws: segregation (monohybrid) and independent assortment (dihybrid, unlinked genes).
• Dominance, recessiveness, genotype vs phenotype, homozygous vs heterozygous.
• Monohybrid cross ratios 3:1 (F₂ phenotypic), dihybrid 9:3:3:1.
• Test cross to determine unknown genotype: cross with homozygous recessive.
• Limitations: linkage, incomplete dominance, codominance, epistasis (extensions preview).

Level 1 — Foundations

Verbal: Mendelian genetics explains inheritance patterns using discrete factors (genes) transmitted from parents to offspring in predictable ratios.

Terminology:

• Allele: Alternative form of a gene.
• Genotype: Genetic constitution (e.g., Tt).
• Phenotype: Observable trait (tall).
• Homozygous: TT or tt; Heterozygous: Tt.

Law of Segregation: Two alleles of a gene separate during gamete formation — each gamete gets one allele.

Law of Independent Assortment: Alleles of different genes assort independently (if on different chromosomes / unlinked).

Level 2 — JEE / NEET depth

Monohybrid cross (Tt × Tt):

• Gametes: T and t each 50%.
• F₂ genotypic 1 TT : 2 Tt : 1 tt.
• F₂ phenotypic 3 dominant : 1 recessive (complete dominance).

Dihybrid (AaBb × AaBb): 16 Punnett combinations → 9:3:3:1 phenotypes if independent.

Test cross: Unknown dominant phenotype × homozygous recessive. All dominant offspring → homozygous dominant; 1:1 → heterozygous.

Probability method: Multiply independent event probabilities — faster than full Punnett for trihybrid.

Deviations (NEET extensions):

• Incomplete dominance → 1:2:1 phenotypic.
• Codominance → both alleles expressed (AB blood group).
• Epistasis → one gene masks another (9:3:4 etc.).

Pedigree basics: Autosomal dominant/recessive patterns from family charts.

Monohybrid cross ratio

Pea plant: Tt (tall) self-cross.

Step 1 — Gametes: ½ T, ½ t from each parent.
Step 2 — Punnett: ¼ TT, ½ Tt, ¼ tt.
Step 3 — Phenotype tall if T_ → ¾ tall, ¼ dwarf.
Step 4 — Ratio 3:1 confirms segregation.

Test cross application

Black coat dog (B dominant). Unknown BB or Bb. Cross with bb brown.

Step 1 — If BB × bb → all Bb black (100% black).
Step 2 — If Bb × bb → ½ Bb black, ½ bb brown (1:1).
Step 3 — Observation distinguishes genotype.
Step 4 — Standard Mendelian diagnostic tool.

• State law of segregation.
• F₂ ratio for Aa × Aa?
• Purpose of test cross?
• Difference genotype and phenotype?
• Stretch: Dihybrid cross gamete types from AaBb parent.

NCERT Chapter 5 link: Mendel's laws explain monohybrid and dihybrid ratios with Punnett squares or probability multiplication. Test cross identifies unknown dominant genotype — cross with homozygous recessive.

Exam connections: 3:1 phenotypic F₂ with complete dominance; 1:2:1 genotypic. Dihybrid 9:3:3:1 when genes unlinked. Deviations: incomplete dominance (1:2:1 phenotypic), codominance (1:2:1 with both expressed), epistasis modified ratios (9:3:4 etc.).

Study strategy: Law of segregation = monohybrid; independent assortment = dihybrid unlinked. Probability method faster for trihybrid: (1/2)ⁿ for gamete types. Pedigree analysis: autosomal recessive skips generations, appears in consanguineous mating.

When studying Mendelian Genetics within Genetics, start by listing every formula and definition on one page without looking at the textbook. Compare your list to NCERT — missing items indicate gaps to fix immediately. Work through at least two NCERT Examples for this section with steps written in full; examiners award method marks even when arithmetic slips.

For board exams (CBSE), long answers benefit from a clear structure: definition → explanation → diagram or formula → example → brief conclusion. Underline key terms. For JEE Main and NEET, prioritise conceptual traps and quick calculation paths; timed mixed quizzes of 10 questions after revision simulate exam pressure.

Cross-topic link: Diagrams and terminology precision matter; link molecular genetics to biotechnology applications chapters.

Spaced revision: Review this note at 1 day, 3 days, and 7 days after first study. Attempt the Quick check questions closed-book, then open the Practice tab for graded reinforcement. Maintain an error log — repeated mistake patterns reveal whether the issue is concept, formula recall, or careless reading.

Diagram and terminology drill: For Biology, redraw key figures from memory and define every labelled part in one sentence. Vocabulary precision prevents mark loss in descriptive answers — use NCERT terms exactly as printed in the textbook.

Revision tip: Link this topic to adjacent Class 12 chapters before attempting mixed practice.

Open the Practice tab for graded questions on Mendelian Genetics.