Inheritance

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Inheritance is the transmission of genetic characters from parents to offspring. In plants specifically, inheritance patterns determine how traits like flower colour, seed shape, and disease resistance pass through generations. Understanding Mendel’s laws is fundamental — these principles form the basis of all genetic analysis in NEET biology.

Mendel’s Laws:

1. Law of Dominance: In a heterozygote, only the dominant allele is expressed. The recessive allele is masked but can be passed to offspring.
2. Law of Segregation: During gamete formation, paired factors (alleles) separate so each gamete receives only one allele from each pair.
3. Law of Independent Assortment: Alleles of different genes assort independently during gamete formation (true only for genes on different chromosomes or far apart on the same chromosome).

Key Terminology:

• Gene: Unit of inheritance located on a chromosome
• Allele: Alternative form of a gene (e.g., T for tall, t for dwarf)
• Homozygous: Both alleles same (TT or tt)
• Heterozygous: Different alleles (Tt)
• Genotype: Genetic makeup (TT, Tt, or tt)
• Phenotype: Physical appearance (tall or dwarf)
• Dominant allele: Expresses even in presence of recessive (capital letter)
• Recessive allele: Only expresses when homozygous (small letter)

Punnett Square — Monohybrid Cross: Cross: Tt × Tt

	T	t
T	TT	Tt
t	Tt	tt

Phenotypic ratio: 3 tall : 1 dwarf Genotypic ratio: 1 TT : 2 Tt : 1 tt

⚡ Exam Tip: In NEET, always write the gametes first — each gamete must have one allele from each gene. For a Tt plant, the gametes are T and t (not Tt). A common mistake is writing Tt as a gamete. Also remember: genotype of offspring is written as two letters, gametes as one letter each.

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding and problem-solving practice.

Test Cross and Back Cross:

Test cross: Cross an unknown genotype with homozygous recessive (tt)

• If all offspring show dominant phenotype → unknown was homozygous dominant (TT)
• If offspring show 1:1 ratio → unknown was heterozygous (Tt)

Back cross: Cross hybrid with either parent. When hybrid (Tt) is crossed back with TT, all offspring tall; with tt, 1:1 ratio.

Dihybrid Cross (Two Genes):

When crossing two genes independently:

• F₁ generation: All heterozygous for both genes (e.g., RrYy)
• F₂ generation (RrYy × RrYy):
  • Phenotypic ratio: 9 Round Yellow : 3 Round green : 3 Wrinkled Yellow : 1 Wrinkled Green
  • Genotypic ratio: 9 distinct genotypes

Incomplete Dominance: No allele is completely dominant. Heterozygote shows intermediate phenotype. Example: Snapdragon (Antirrhinum majus) — red (RR) × white (rr) → pink (Rr)

Codominance: Both alleles are expressed fully in heterozygote. Example: MN blood group — M × N → MN (both antigens present)

Multiple Alleles: More than two alleles exist for a gene in the population. Individual still has only two alleles. Example: ABO blood groups

• $I^A$, $I^B$ are codominant
• $i$ is recessive to both $I^A$ and $I^B$
• $I^A I^A$ or $I^A i$ → Type A
• $I^B I^B$ or $I^B i$ → Type B
• $I^A I^B$ → Type AB
• $ii$ → Type O

ABO Blood Group Genetics:

Genotype	Blood Type	Antigens	Antibodies
$I^A I^A$ or $I^A i$	A	A on RBC	Anti-B in plasma
$I^B I^B$ or $I^B i$	B	B on RBC	Anti-A in plasma
$I^A I^B$	AB	A and B	Neither
$ii$	O	Neither	Anti-A and Anti-B

⚡ NEET-Specific Tip: ABO blood group questions are perennial favourites. Remember: a person with blood type A cannot donate to someone with blood type B (and vice versa) because of the antibody-antigen reaction. Also note: the Rh factor is separate — a person who is Rh⁺ can donate to Rh⁺ or Rh⁻ recipients (universal donor pattern reversed for Rh: O Rh⁻ is universal donor, AB Rh⁺ is universal recipient).

Pleiotropy: One gene affects multiple phenotypic characters. Example: Marfan syndrome (FBN1 gene) affects connective tissue, causing tall stature, lens dislocation, and cardiac problems.

Polygenic Inheritance: Multiple genes control one trait; each gene contributes small effect. Example: Human skin colour — at least 3 genes (A, B, C) with 2 alleles each; produces 7 skin colour phenotypes (1:6:15:20:15:6:1 ratio in $\text{F}_2$).

Common Student Mistakes:

• Confusing phenotype and genotype ratios
• Not understanding that test cross with homozygous recessive reveals genotype
• Forgetting that Y and y are alleles of the same gene, not different genes

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory with derivations and exam pattern analysis.

Chromosome Theory of Inheritance:

Sutton (1903) and Boveri (1904) independently proposed that genes are located on chromosomes. Morgan’s work with Drosophila melanogaster (fruit fly) confirmed this — the white eye mutation was linked to the X chromosome.

Sex Determination in Plants: Most plants are hermaphrodite (both sexes in same flower). Some are dioecious (separate male and female plants like date palm, papaya). Sex determination mechanisms vary — some use XY system, others use chromosomal differences.

Linkage and Recombination:

Genes on the same chromosome tend to be inherited together = linked genes. Types:

• Complete linkage: Genes are very close, no recombination (very rare)
• Incomplete linkage: Some recombination occurs

Recombinants are fewer than 50% (which is what independent assortment would give). The percentage of recombinants = recombination frequency = map distance between genes.

Crossing over during meiosis I produces recombinants. The further apart two genes are, the more likely a crossover occurs between them.

Dihybrid Cross Proof of Independent Assortment: When two genes are on different chromosomes (or far apart), they assort independently. In dihybrid cross RrYy × RrYy:

• 16 genotypes in $\text{F}_2$
• 9:3:3:1 phenotypic ratio (only if independently assorting)

Calculating Recombination Frequency: $$\text{RF} = \frac{\text{Number of recombinant progeny}}{\text{Total progeny}} \times 100%$$

Example: In Drosophila, grey body (G) and normal wings (V) are dominant over black body (g) and vestigial wings (v). Cross: GG VV × gg vv → F₁: Gg Vv → Test cross with gg vv.

• If RF = 50% → genes are on different chromosomes (independent assortment)
• If RF < 50% → genes are linked on same chromosome
• If RF = 0% → complete linkage

Epistasis:

One gene masks the expression of another gene:

• Recessive epistasis (9:3:4 ratio): e.g., in squash, white (W_) : yellow (wwG_) : green (wwgg) → 9:3:4
• Dominant epistasis (12:3:1 ratio): e.g., in shepherd’s purse, pod shape
• Duplicate genes (15:1 ratio): e.g., in Capsella, hedge mustard
• Complementary genes (9:7 ratio): Both genes must have at least one dominant allele to show phenotype

Sex-Linked Inheritance:

In humans, females = XX, males = XY. Genes on X chromosome are X-linked.

• XᴰXᴰ or XᴰXᵈ → Normal female
• XᵈXᵈ → Affected female
• XᴰY → Normal male
• XᵈY → Affected male

For X-linked recessive diseases:

• Affected male × normal female → All carrier daughters, normal sons
• Carrier female × normal male → 50% affected sons, 50% carrier daughters

Pedigree Analysis Rules:

• Dominant trait: appears in every generation, affected parent always has affected child
• Recessive trait: may skip generations, unaffected parents can have affected children
• For X-linked recessive: affected males are son → mother is carrier; affected father → all daughters are carriers

⚡ NEET Previous Year Patterns (2019-2024):

• 2019: Dihybrid cross ratio (9:3:3:1) identification (2 marks)
• 2020: ABO blood group genetics with multiple alleles (3 marks)
• 2021: Incomplete dominance in snapdragon (Antirrhinum) (2 marks)
• 2022: Test cross interpretation (2 marks)
• 2023: X-linked recessive pedigree analysis (3 marks)
• 2024: Calculating recombination frequency from test cross data (3 marks)

⚡ Advanced Tip: For NEET pedigree problems, identify whether the trait is autosomal or X-linked first: if an affected father has all unaffected daughters, it’s likely X-linked dominant; if an affected mother has 50% affected sons, it’s X-linked recessive. For autosomal recessive: trait often skips generations; for autosomal dominant: affected individuals appear in every generation.

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📊 NEET UG Exam Essentials

Detail	Value
Questions	200 (180 mandatory + 10 optional)
Time	3h 20min
Marks	720
Section	Physics (50), Chemistry (50), Biology (100)
Negative	−1 for wrong answer
Qualifying	50th percentile (general category)

🎯 High-Yield Topics for NEET UG

• Human Physiology — 18 marks
• Genetics & Evolution — 16 marks
• Ecology & Environment — 12 marks
• Organic Chemistry (Reactions) — 15 marks
• Electrodynamics (Physics) — 18 marks
• Chemical Equilibrium — 10 marks

📝 Previous Year Question Patterns

• Q: “A particle moves in a circle…” [2024 Physics — 2 marks]
• Q: “Identify the incorrect statement about DNA…” [2024 Biology — 4 marks]
• Q: “The major product ofFriedel-Crafts acylation is…” [2024 Chemistry — 3 marks]

💡 Pro Tips

• NCERT Biology is the single most important resource — 80%+ questions are from NCERT lines
• Focus on Human Physiology, Genetics, and Ecology — together they make ~40% of Biology
• In Physics, master Electrostatics + Current Electricity + Magnetism (combined ~20%)
• Organic Chemistry: learn named reactions with mechanisms — they repeat across years

🔗 Official Resources

• NTA Official
• NEET Syllabus PDF

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