Principles of Inheritance and Variation - Complete NEET Biology Notes

Genetics is the highest weightage chapter in NEET Biology, contributing 8-12 questions annually. This comprehensive guide covers all NCERT concepts essential for NEET 2026.

Mendel and His Work

Why Pea Plant?

Advantage	Description
Contrasting traits	7 pairs of clearly visible differences
Annual plant	Short generation time
Self-pollination	Easy to maintain pure lines
Cross-pollination	Can be artificially done
Large number of seeds	Statistical analysis possible

Seven Contrasting Traits Studied

Trait	Dominant	Recessive
Seed shape	Round (R)	Wrinkled (r)
Seed color	Yellow (Y)	Green (y)
Pod shape	Inflated (I)	Constricted (i)
Pod color	Green (G)	Yellow (g)
Flower color	Purple (P)	White (p)
Flower position	Axial (A)	Terminal (a)
Stem height	Tall (T)	Dwarf (t)

Mendel's Laws of Inheritance

1. Law of Dominance

Statement: When two organisms with contrasting characters are crossed, only one character appears in F1 (dominant), while the other remains hidden (recessive).

Key Points:

Factors occur in pairs (alleles)
In heterozygote, dominant allele masks recessive
Explains 3:1 ratio in F2

2. Law of Segregation (Purity of Gametes)

Statement: During gamete formation, the two alleles of a gene separate so that each gamete receives only one allele.

Monohybrid Cross Example:

P: TT × tt (Tall × Dwarf)
F1: All Tt (Tall)
F2: TT : Tt : Tt : tt = 1:2:1 (genotypic)
F2 Phenotypic ratio: 3 Tall : 1 Dwarf

Test Cross: Cross with homozygous recessive to determine genotype.

Tt × tt → 1 Tall : 1 Dwarf
TT × tt → All Tall

3. Law of Independent Assortment

Statement: Alleles of different genes assort independently of each other during gamete formation, provided genes are on different chromosomes.

Dihybrid Cross Example:

P: RRYY × rryy (Round Yellow × Wrinkled Green)
F1: RrYy (Round Yellow)
F2 Ratio: 9:3:3:1

Phenotype	Ratio	Genotypes
Round Yellow	9	R_Y_
Round Green	3	R_yy
Wrinkled Yellow	3	rrY_
Wrinkled Green	1	rryy

Extensions of Mendelian Genetics

Incomplete Dominance

Neither allele is completely dominant; heterozygote shows intermediate phenotype.

Example: Snapdragon flower color

RR = Red, rr = White, Rr = Pink
F2 ratio: 1 Red : 2 Pink : 1 White

Co-dominance

Both alleles express equally in heterozygote.

Example: ABO blood groups

I^A I^A = Type A
I^B I^B = Type B
I^A I^B = Type AB (both antigens present)
ii = Type O

Multiple Alleles

More than two alleles exist for a gene in a population.

Example: ABO blood groups (3 alleles: I^A, I^B, i)

Pleiotropy

Single gene affects multiple traits.

Example: Sickle cell anemia - single mutation causes multiple effects:

Abnormal hemoglobin
Sickle-shaped RBCs
Anemia
Organ damage

Epistasis

One gene masks the expression of another gene.

Example: Coat color in dogs

B_E_ = Black
bbE_ = Brown
B_ee = Golden
bbee = Golden
Ratio: 9:3:4 (instead of 9:3:3:1)

Polygenic Inheritance

Multiple genes control one trait; shows continuous variation.

Example: Skin color in humans (3+ genes)

Many genotypes possible
Bell-shaped distribution curve

Chromosomal Theory of Inheritance

Proposed by: Sutton and Boveri (1902)

Key Points

Chromosomes occur in pairs (homologous pairs)
Genes are located on chromosomes
Homologous chromosomes separate during meiosis (like alleles)
Chromosomes assort independently during meiosis

Linkage

Genes on the same chromosome tend to inherit together.

Types:

Complete linkage: No crossing over (rare)
Incomplete linkage: Some crossing over

Coupling (Cis): Dominant alleles on same chromosome (AB/ab) Repulsion (Trans): Dominant alleles on different homologs (Ab/aB)

Recombination Frequency

Measures distance between linked genes
Higher crossing over = greater distance = higher RF
RF = (Recombinant offspring / Total offspring) × 100
1% RF = 1 centimorgan (cM) = 1 map unit

Sex Determination

XX-XY System (Humans, most mammals)

Sex	Chromosomes	Gametes
Female	44 + XX	All carry X
Male	44 + XY	Half X, half Y

Sex of offspring determined by father's sperm.

XX-XO System (Grasshoppers)

Female: XX
Male: XO (only one sex chromosome)

ZW-ZZ System (Birds, some insects)

Female: ZW (heterogametic)
Male: ZZ

Haplodiploidy (Bees)

Female: Diploid (from fertilized egg)
Male: Haploid (from unfertilized egg)

Sex-Linked Inheritance

X-Linked Traits

Characteristics:

More common in males
Criss-cross inheritance (father → daughter → grandson)
Carrier females possible

Examples:

Color blindness
Hemophilia
Duchenne muscular dystrophy

Color Blindness Example

Cross	Offspring
X^C X^C × X^C Y	All normal
X^C X^c × X^C Y	1 normal female : 1 carrier female : 1 normal male : 1 colorblind male
X^c X^c × X^C Y	All females carriers : All males colorblind

Hemophilia

Blood clotting disorder
X-linked recessive
Called "Royal disease" (British royal family)

Y-Linked (Holandric) Traits

Only males affected
Father to all sons transmission
Example: Hypertrichosis (hairy ear rims)

Chromosomal Disorders

Aneuploidy

Abnormal number of chromosomes.

Type	Definition
Monosomy	2n - 1 (one less)
Trisomy	2n + 1 (one extra)
Nullisomy	2n - 2
Tetrasomy	2n + 2

Autosomal Disorders

Down Syndrome (Trisomy 21)

Feature	Description
Karyotype	47, +21
Cause	Non-disjunction in meiosis
Risk factor	Advanced maternal age
Features	Intellectual disability, flat face, short stature, congenital heart defects

Sex Chromosomal Disorders

Turner Syndrome (45, X)

Feature	Description
Karyotype	45, X (monosomy X)
Sex	Female
Features	Short stature, webbed neck, sterile, underdeveloped ovaries

Klinefelter Syndrome (47, XXY)

Feature	Description
Karyotype	47, XXY
Sex	Male
Features	Tall, gynecomastia, sterile, small testes

Super Female (47, XXX)

Feature	Description
Karyotype	47, XXX
Sex	Female
Features	Usually normal, may have reduced fertility

Mendelian Disorders (Single Gene)

Autosomal Dominant

Disorder	Affected Gene/Protein
Huntington's disease	Huntingtin protein
Polydactyly	Extra fingers/toes
Brachydactyly	Short fingers

Autosomal Recessive

Disorder	Cause	Features
Phenylketonuria (PKU)	Phenylalanine hydroxylase deficiency	Mental retardation if untreated
Sickle cell anemia	Abnormal hemoglobin (HbS)	Sickle-shaped RBCs, anemia
Thalassemia	Reduced/absent globin chains	Severe anemia
Cystic fibrosis	CFTR protein defect	Thick mucus, lung infections
Albinism	Tyrosinase deficiency	No melanin pigment

Sickle Cell Anemia

Point mutation: GAG → GUG (Glutamic acid → Valine)
Position: 6th amino acid of β-globin
Heterozygotes (HbA HbS): Carrier, sickle cell trait
Provides malaria resistance (balanced polymorphism)

Pedigree Analysis

Symbols

Square: Male
Circle: Female
Filled: Affected
Half-filled: Carrier
Horizontal line: Mating
Vertical line: Offspring

Identifying Inheritance Pattern

Pattern	Characteristics
Autosomal dominant	Affected in every generation, both sexes equally
Autosomal recessive	May skip generations, both sexes equally
X-linked recessive	More males affected, never father to son
X-linked dominant	Affected father → all daughters affected

Previous Year NEET Questions

Q1 (NEET 2023): A cross between two tall plants produced 75 tall and 25 dwarf plants. The genotype of parents would be:

(a) TT × TT
(b) Tt × tt
(c) Tt × Tt ✓
(d) TT × Tt

Q2 (NEET 2022): Which of the following is an example of co-dominance?

(a) Skin color in humans
(b) Flower color in snapdragon
(c) ABO blood groups ✓
(d) Flower color in pea

Q3 (NEET 2021): Down syndrome is caused by:

(a) Monosomy
(b) Trisomy 21 ✓
(c) Sex chromosome abnormality
(d) Translocation only

Q4 (NEET 2020): In a dihybrid cross, F2 generation ratio is:

(a) 3:1
(b) 1:1
(c) 9:3:3:1 ✓
(d) 1:2:1

Q5 (NEET 2019): Color blindness is more common in males because:

(a) Y chromosome carries the gene
(b) X chromosome carries the gene and males have only one X ✓
(c) Males have two X chromosomes
(d) It is Y-linked trait

Quick Revision Points

Law of Segregation: Alleles separate during gamete formation
Law of Independent Assortment: Only for genes on different chromosomes
Monohybrid F2 ratio: 3:1 (phenotypic), 1:2:1 (genotypic)
Dihybrid F2 ratio: 9:3:3:1
Incomplete dominance: 1:2:1 (phenotypic)
Co-dominance: Both alleles expressed (ABO blood groups)
Multiple alleles example: ABO blood groups (I^A, I^B, i)
Pleiotropy example: Sickle cell anemia
Epistasis: One gene masks another
Sex determination: Father determines sex in humans
X-linked recessive: More common in males
Down syndrome: Trisomy 21 (47 chromosomes)
Turner syndrome: 45, X (female)
Klinefelter syndrome: 47, XXY (male)
Sickle cell mutation: Glu → Val at position 6 of β-globin

FAQs

Q: Why is the 9:3:3:1 ratio not always observed? A: This ratio assumes: (1) genes are on different chromosomes (no linkage), (2) no gene interactions (epistasis), (3) complete dominance. Deviations occur when these conditions aren't met.

Q: Why is color blindness more common in males? A: The gene for color vision is on the X chromosome. Males (XY) have only one X, so a single mutant allele causes the condition. Females (XX) need mutations on both X chromosomes to be affected.

Q: How does non-disjunction cause chromosomal disorders? A: Non-disjunction is failure of chromosomes to separate during meiosis. This produces gametes with extra or missing chromosomes, leading to conditions like Down syndrome (trisomy 21) when fertilized.

Q: Why do carriers of sickle cell anemia have malaria resistance? A: Carriers (HbA HbS) have some sickle-shaped RBCs under low oxygen. The malaria parasite cannot survive well in these cells, providing selective advantage in malaria-endemic regions.

Q: What is the difference between linkage and crossing over? A: Linkage keeps genes on the same chromosome together during inheritance. Crossing over (during meiosis) breaks this linkage by exchanging segments between homologous chromosomes, creating recombinant offspring.

Key Takeaways

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