Neet biology : principles of inheritance and variation short Notes 2027, Chapter 4, NCERT+PYQ

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I am a  NEET Dropper. Today I'm sharing Complete Notes of principles of Inheritance and Variation Class 12 Biology Chapter 4
*Short Notes +  for NEET -

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### *1. Mendel & His Experiments*
*Why Pea Plant?* 

1. 7 contrasting traits 

2. Self-pollinating but cross-pollination possible 

3. Short life cycle, large progeny 

*Diagram: Mendel’s 7 Contrasting Traits*

1. Seed Shape     → Round (R) vs Wrinkled (r)

2. Seed Color     → Yellow (Y) vs Green (y)

3. Flower Color   → Violet (V) vs White (v)

4. Pod Shape      → Inflated (I) vs Constricted (i)

5. Pod Color      → Green (G) vs Yellow (g)

6. Flower Pos.    → Axial (A) vs Terminal (a)

7. Stem Height    → Tall (T) vs Dwarf (t)

*Trick*: *R*ohan *Y*esterday *V*isited *I*ndia *P*ark *F*or *S*ightseeing

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### *2. Mendel’s Laws of Inheritance*

*A. Law of Dominance* 
In heterozygotes, only one allele expresses itself. 
Ex: Tt = Tall. T is dominant over t.

*B. Law of Segregation - Monohybrid Cross* 
Alleles separate during gamete formation. 
*Diagram: TT x tt*
Parents:     TT (Tall)   x   tt (Dwarf)
Gametes:        T                 t
F1:                 Tt → All Tall

F1 Selfing:  Tt  x  Tt
Gametes:    T, t     T, t
F2:  TT : Tt : Tt : tt
     1  : 2  :     1
Phenotypic Ratio = 3 Tall : 1 Dwarf
Genotypic Ratio  = 1 : 2 : 1
*Test Cross*: Tt x tt → 1 Tall : 1 Dwarf. Used to find unknown genotype.

*C. Law of Independent Assortment - Dihybrid Cross* 
Alleles of 2 genes assort independently. 
*Diagram: RRYY x rryy*
P:  RRYY (Round Yellow) x rryy (Wrinkled Green)
F1:          RrYy → All Round Yellow

F2 Ratio: 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green
          = 9 : 3 : 3 : 1
*NEET Point*: 9:3:3:1 valid only if genes are unlinked.

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### *3. Deviations from Mendel*
Concept Example F2 Ratio Key Point
**Incomplete Dominance** Snapdragon Flower 1 Red : 2 Pink : 1 White Rr = Intermediate
**Co-dominance** ABO Blood Group IAIB = AB blood Both alleles express
**Multiple Alleles** ABO: IA, IB, i 4 Phenotypes possible Individual has only 2 alleles
**Pleiotropy** Phenylketonuria 1 gene → many effects Mental + hair issues
**Polygenic Inheritance** Human skin color Continuous variation Bell curve graph
*Diagram: ABO Blood Groups*
Genotype        →  Blood Group  →  Can Receive From
IAIA, IAi       →  A            →  A, O
IBIB, IBi       →  B            →  B, O
IAIB            →  AB           →  A, B, AB, O  ← Universal Recipient
ii              →  O           →  O only       ← Universal Donor
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### *4. Chromosomal Theory + Linkage - Morgan’s Work*

*Chromosomal Theory*: Sutton & Boveri. Genes are on chromosomes. Both segregate together.

*Linkage & Recombination* 
*Diagram: Morgan’s Drosophila Experiment*
Cross: Yellow body, White eye ♀  x  Wild type ♂
Result: Parental types = 98.7% , Recombinants = 1.3%
Conclusion: Genes close on chromosome = Linked = Less recombination
*Map Distance*: 1% Recombination = 1 cM = 1 map unit. 
*Trick*: *Close genes = Less cross over = More linkage*

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### *5. Sex Determination*
Type Male Female Example
**XX-XY** XY XX Humans, Drosophila
**XX-XO** XO XX Grasshopper
**ZZ-ZW** ZZ ZW Birds, Butterfly
**Haplo-diploid** n 2n Honeybee: Male from unfertilized egg
*Diagram: Sex Determination in Humans*
Father (XY) → Sperms: 50% X, 50% Y
Mother (XX) → Eggs: 100% X
Offspring: XX = Girl (50%), XY = Boy (50%)
So father determines sex, not mother.
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### *6. Mutation & Genetic Disorders*

*A. Mutation*: Sudden change in DNA. 
*Point Mutation Ex*: Sickle Cell Anaemia → GAG to GUG → Glu to Val in β-globin chain.

*B. Pedigree Symbols*
□ = Male, ○ = Female
■ ● = Affected, □ ○ = Normal
Horizontal line = Mating, Vertical = Children
*How to identify*: 

1. *Autosomal Dominant*: Appears every generation, M=F 

2. *Autosomal Recessive*: Skips generations, parents normal child affected 

3. *X-Linked Recessive*: More males affected, carrier mother to son

*C. Main Disorders Table*
Disorder Type Chromosome Symptom
**Haemophilia** X-linked Recessive X Blood not clot
**Color Blindness** X-linked Recessive X No red-green vision
**Sickle Cell Anaemia** Autosomal Recessive 11 RBC sickle shaped
**Phenylketonuria** Autosomal Recessive 12 Mental retardation
**Down’s Syndrome** Aneuploidy Trisomy 21 47 chromosomes, flat face
**Klinefelter’s** Aneuploidy XXY 47, Male sterile, tall
**Turner’s** Aneuploidy XO 45, Female sterile, short
*Diagram: Aneuploidy Karyotypes*
Normal Female: 44A + XX = 46
Normal Male:   44A + XY = 46
Down’s:        45A + XX/XY = 47  → Extra 21st chromosome
Klinefelter’s: 44A + XXY = 47    → Extra X in male
Turner’s:      44A + XO = 45     → Missing X in female
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### *7. NEET Last-Minute Checklist: M.E.N.D.E.L*

*M*endel’s Laws → *E*xceptions to Mendel → *N*on-Mendelian → *D*isorders → *E*pistasis/Linkage → *L*aws of Segregation & Assortment

*3 High-Yield PYQ Types:*

1. *Q*: In ABO blood group, a man with B marries woman with A. Child is O. Find parent genotypes. 

   *Ans*: IBi x IAi → ii child possible. Both heterozygous.

2. *Q*: Why 9:3:3:1 ratio deviates? 

   *Ans*: Linkage, epistasis, lethal genes. Assortment not independent.

3. *Q*: Female carrier of haemophilia marries normal male. % of sons affected? 

   *Ans*: XHXh x XHY → 50% sons affected, 50% daughters carrier.