Inheritance

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Inheritance in zoology refers to how genetic traits pass from parents to offspring through genes located on chromosomes. In animals, this process follows Mendel’s laws but with additional complexity due to sex determination systems, sex-linked inheritance, and the role of environmental factors in phenotyp. The genome of an organism contains thousands of genes that determine everything from eye colour to metabolic processes.

Key Concepts:

• Gene: Functional unit of heredity located at a specific locus on a chromosome
• Allele: Alternate forms of a gene (e.g., gene for flower colour has allele for red and allele for white)
• Dominant allele: Expresses in heterozygote (capital letter: B)
• Recessive allele: Only expresses when homozygous (small letter: b)
• Homozygous: Both alleles identical (BB or bb)
• Heterozygous: Different alleles (Bb)
• Genotype: Genetic makeup (BB, Bb, or bb)
• Phenotype: Observable trait (e.g., red flowers)

Mendel’s Laws in Animals:

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

Punnett Square — Monohybrid Cross: Cross: Bb × Bb

	B	b
B	BB	Bb
b	Bb	bb

Phenotypic ratio: 3 dominant : 1 recessive Genotypic ratio: 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive

⚡ Exam Tip: In NEET, the most common error is students writing gametes as having two alleles (like “Bb”). Remember: a gamete can only contain ONE allele per gene. A Bb individual produces gametes B and b — not Bb.

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🟡 Standard — Regular Study (2d–2mo)

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

Test Cross: Cross unknown genotype with homozygous recessive (bb):

• All offspring dominant phenotype → unknown was BB (homozygous dominant)
• 50% dominant : 50% recessive → unknown was Bb (heterozygous)

This is a crucial diagnostic tool in genetics. Breeders use it to determine if an animal with dominant phenotype is pure-bred (homozygous) or hybrid (heterozygous).

Incomplete Dominance: Neither allele is dominant — heterozygote shows intermediate phenotype. Example in animals: Andalusian chickens — black (BB) × white (bb) → blue (Bb). This gives 1:2:1 phenotypic ratio identical to genotypic ratio.

Codominance: Both alleles are fully expressed in heterozygote. Example: ABO blood groups in humans (discussed below).

Multiple Alleles — ABO Blood Group System:

Three alleles exist in human population: $I^A$, $I^B$, and $i$. Each person has only two of these.

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

⚡ Exam Tip: A person with blood type A has anti-B antibodies in their plasma — if they receive type B blood, their antibodies will attack the B antigen on donor RBCs, causing agglutination (clumping). This is why transfusion must be compatible. Also note: O blood type (ii) is the universal donor because it has no A or B antigens to react with. AB (I^A I^B) is the universal recipient because they have no anti-A or anti-B antibodies.

Rh Factor: Separate genetic system. Rh⁺ (dominant) or Rh⁻ (recessive). A Rh⁻ mother carrying a Rh⁺ baby can develop anti-Rh antibodies after first delivery (sensitisation), causing haemolytic disease of the newborn in subsequent pregnancies.

Sex Determination in Animals:

Organism	Sex Chromosomes	Method
Humans	XX (♀), XY (♂)	XY system
Birds	ZZ (♂), ZW (♀)	ZW system
Grasshopper	XX (♀), XO (♂)	XO system
Honeybee	Diploid (♀), Haploid (♂)	Haplodiploidy

In humans, the Y chromosome carries the SRY gene which triggers male development. Without Y chromosome, default development is female.

Sex-Linked Inheritance in Humans:

Genes on X chromosome (X-linked):

• Xᴰ = normal allele; Xᵈ = disease allele
• Males are hemizygous (have only one X) — express whichever allele they carry
• Carrier females: XᴰXᵈ (heterozygous)

Examples:

• Colour blindness (red-green): X-linked recessive
• Haemophilia (X-linked recessive): Factor VIII or IX deficiency
• Duchenne muscular dystrophy (X-linked recessive)
• Turner’s syndrome (XᴰO): 45,X — sterile female
• Klinefelter’s syndrome (XᴰXᴰY): 47,XXY — sterile male

Common Student Mistakes:

• Confusing phenotype and genotype ratio calculations
• Forgetting that sex-linked recessive traits affect males more frequently (they only need one copy)
• Mixing up autosomal and sex-linked inheritance patterns

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory with derivations and exam pattern analysis.

Linkage and Crossing Over:

Genes located on the same chromosome tend to be inherited together = linked genes. The closer two genes are, the less likely a crossover between them.

Crossing over during prophase I of meiosis exchanges segments between non-sister chromatids of homologous chromosomes, producing recombinants.

Recombination frequency (RF): $$\text{RF} = \frac{\text{Number of recombinant progeny}}{\text{Total progeny}} \times 100%$$

RF is proportional to map distance between genes. 1% RF = 1 centimorgan (cM).

Example: In Drosophila, grey body (G) and long wings (V) are dominant over black body (g) and vestigial wings (v). When F₁ heterozygote (Gg Vv) is test-crossed with gg vv:

• Parental types: Grey long (G_V_) and black vestigial (ggvv)
• Recombinant types: Grey vestigial (Ggvv) and black long (ggVv)

Morgan’s Drosophila Experiments: Thomas Hunt Morgan (1910) discovered sex linkage by studying white eye mutation in Drosophila melanogaster:

• White eye (w) is X-linked recessive; wild type is red (w⁺)
• P: red-eyed female × white-eyed male → F₁: all red eyes (females Xᴅˣʷ, males XᴅY)
• F₂: red females : red males : white males = 1 : 1 : 1

This proved genes are on chromosomes and sex-linked inheritance patterns follow chromosome inheritance.

Chromosome Theory of Inheritance:

Sutton (1903) and Boveri (1904) independently proposed that:

1. Chromosomes contain genes
2. Chromosomes pair during meiosis
3. Homologous chromosomes separate during anaphase I
4. Genes on different chromosomes assort independently

Pedigree Analysis:

For autosomal recessive traits:

• Affected individuals usually have unaffected parents (trait “skips generations”)
• Two heterozygous carriers have 25% chance of affected offspring
• Consanguinity increases risk (shared ancestry)

For autosomal dominant traits:

• Every affected person has at least one affected parent
• Vertical transmission (no skipping)
• Approximate 50% transmission to offspring if parent is heterozygous

For X-linked recessive:

• Primarily affects males
• Affected male × normal female → all carrier daughters, no affected sons
• Carrier female × normal male → 50% chance carrier daughters, 50% affected sons

For X-linked dominant:

• More females affected than males
• Affected male × normal female → all daughters affected
• Carrier female × normal male → 50% affected offspring (both sexes)

Epistasis in Animals:

One gene masks another gene’s expression:

• Recessive epistasis: Both homozygous recessive for one gene mask another gene (9:3:4 ratio in F₂)
• Dominant epistasis: At least one dominant allele at one locus masks another gene (12:3:1 ratio)
• Duplicate genes: Two genes produce same phenotype (15:1 ratio)
• Complementary genes: Both genes must have at least one dominant allele for normal phenotype (9:7 ratio)

Polygenic Inheritance:

Multiple genes contribute to one trait. Each gene has small additive effect. Human examples:

• Height: ~400-500 genes involved
• Skin colour: ~6 genes
• Eye colour: ~16 genes Continuous variation produces bell-curve distribution.

Lethal Genes:

Genes that cause death when homozygous (before or after birth). Examples:

• Manx cat (heterozygous = tailless, homozygous = fatal)
• Huntington’s disease in humans (dominant lethal, but symptom appears after reproductive age)

Cytoplasmic/Mitochondrial Inheritance:

Genes in mitochondrial DNA are maternally inherited (egg contributes mitochondria, sperm contributes negligible). Examples:

• Mitochondrial myopathies
• Leber’s hereditary optic neuropathy (LHON)

NEET Previous Year Patterns (2019-2024):

• 2019: Colour blindness pedigree analysis identifying X-linked recessive (3 marks)
• 2020: ABO blood group genetics with multiple alleles including Rh compatibility (3 marks)
• 2021: Dihybrid cross ratio (9:3:3:1) and identifying linkage vs independent assortment (3 marks)
• 2022: Calculating recombination frequency from test cross data in Drosophila (3 marks)
• 2023: Incomplete dominance in Andalusian fowl or snapdragon (2 marks)
• 2024: Mitochondrial inheritance and cytoplasmic male sterility in plants (2 marks)

⚡ Advanced Tip: For pedigree analysis problems, count carefully: if an affected daughter has two unaffected parents, the trait CANNOT be autosomal dominant (affected daughter needs at least one affected parent) — it must be autosomal recessive or X-linked recessive. If ALL daughters of an affected father are affected, it’s likely X-linked dominant. For autosomal recessive traits with no male-to-male transmission, look for “consanguinity” clues or affected individuals with unaffected parents.

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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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