NEET Biology — Principles of Inheritance and Variation…

Chapter Overview & Weightage

Principles of Inheritance is one of the highest-weightage chapters in NEET Biology. It covers Mendel’s laws, deviations from Mendelism (incomplete dominance, codominance, multiple alleles), sex-linked inheritance, and chromosomal disorders. Expect both theory questions and cross-based numerical problems.

This chapter carries 6-8% weightage in NEET — that’s 5-6 questions per paper. Mendelian crosses, sex-linked inheritance, and chromosomal disorders are perennial favourites. At least one numerical problem (phenotypic ratio prediction) appears every year.

Key Concepts You Must Know

Tier 1 (Core)

Mendel’s laws: Law of Dominance, Law of Segregation, Law of Independent Assortment
Monohybrid cross: $F_2$ ratio = 3:1 (phenotypic), 1:2:1 (genotypic)
Dihybrid cross: $F_2$ ratio = 9:3:3:1
Test cross: cross with homozygous recessive to determine genotype of dominant phenotype
Incomplete dominance: $F_2$ ratio = 1:2:1 (e.g., snapdragon flower colour)
Codominance: both alleles express equally (e.g., ABO blood group — $I^A I^B$ = AB)

Tier 2 (Frequently tested)

Multiple alleles: ABO blood group system ( $I^A$ , $I^B$ , $i$ alleles)
Sex-linked inheritance: X-linked recessive traits (colour blindness, haemophilia) — carrier females, affected males
Sex determination: XX-XY (humans), ZW-ZZ (birds), XX-XO (grasshopper)
Linkage and recombination: linked genes don’t assort independently, recombination frequency = genetic distance

Tier 3 (Occasionally tested)

Chromosomal disorders: Down syndrome (trisomy 21), Turner syndrome (45, XO), Klinefelter syndrome (47, XXY)
Pedigree analysis: autosomal dominant, autosomal recessive, X-linked patterns
Pleiotropy: one gene affects multiple phenotypic traits (e.g., sickle cell anaemia)

Important Formulas

Cross Type	$F_2$ Phenotypic Ratio	Condition
Monohybrid (complete dominance)	3:1	Dominant:Recessive
Monohybrid (incomplete dominance)	1:2:1	AA:Aa:aa (all different phenotypes)
Dihybrid	9:3:3:1	Both genes independently assort
Test cross (heterozygous)	1:1	Monohybrid test cross
Test cross (dihybrid heterozygous)	1:1:1:1	Dihybrid test cross
Complementary genes	9:7	Two genes needed for one phenotype
Epistasis (recessive)	9:3:4	One gene masks the other

Genotype	Blood Group	Antigens	Antibodies
$I^A I^A$ or $I^A i$	A	A	Anti-B
$I^B I^B$ or $I^B i$	B	B	Anti-A
$I^A I^B$	AB	A and B	None
$ii$	O	None	Anti-A and Anti-B

$I^A$ and $I^B$ are codominant to each other but both are dominant over $i$ .

Disorder	Karyotype	Features
Down syndrome	47, +21 (trisomy 21)	Mental retardation, short stature, broad face
Turner syndrome	45, XO	Female, short stature, infertile, webbed neck
Klinefelter syndrome	47, XXY	Male, tall, gynaecomastia, infertile
Patau syndrome	47, +13 (trisomy 13)	Cleft lip/palate, severe defects
Edwards syndrome	47, +18 (trisomy 18)	Severe cardiac defects

For NEET crosses, always write the genotype first, then make a Punnett square. Even if you can do it mentally, writing it out prevents silly errors. For sex-linked problems, use $X^H X^h$ notation (superscript on X) to track the alleles clearly.

Solved Previous Year Questions

PYQ 1 — NEET 2024

Problem: A colour-blind man marries a carrier woman. What fraction of their sons will be colour-blind?

Solution:

Father: $X^c Y$ (colour-blind) Mother: $X^C X^c$ (carrier)

Cross:

	$X^C$	$X^c$
$X^c$	$X^C X^c$ (carrier daughter)	$X^c X^c$ (colour-blind daughter)
$Y$	$X^C Y$ (normal son)	$X^c Y$ (colour-blind son)

Sons: 1 normal ( $X^C Y$ ) : 1 colour-blind ( $X^c Y$ )

Answer: 1/2 (50%) of sons will be colour-blind

PYQ 2 — NEET 2023

Problem: In a monohybrid cross between Tt and Tt, the phenotypic ratio in $F_2$ is:

(A) 1:1 (B) 1:2:1 (C) 3:1 (D) 9:3:3:1

Solution:

$Tt \times Tt$ → $TT : Tt : tt = 1:2:1$ (genotypic ratio)

With complete dominance, $TT$ and $Tt$ show dominant phenotype, $tt$ shows recessive.

Phenotypic ratio: 3:1 (3 dominant : 1 recessive)

Answer: (C) 3:1

PYQ 3 — NEET 2022

Problem: Klinefelter syndrome has the karyotype:

(A) 45, XO (B) 47, XXY (C) 47, +21 (D) 47, XXX

Solution:

45, XO: Turner syndrome (female, monosomy X)
47, XXY: Klinefelter syndrome (male with extra X, infertile, gynaecomastia)
47, +21: Down syndrome (trisomy 21)
47, XXX: Super female / Triple X syndrome

Answer: (B) 47, XXY

Turner = one X missing (45, XO) = female. Klinefelter = one X extra (47, XXY) = male. The presence of Y chromosome determines maleness regardless of how many X chromosomes are present.

Difficulty Distribution

Difficulty	% of Questions	What to Expect
Easy	35%	Mendelian ratios, blood group genetics, disorder karyotypes
Medium	45%	Sex-linked crosses, incomplete dominance, test cross reasoning
Hard	20%	Pedigree analysis, epistasis ratios, complex crosses

Expert Strategy

Week 1: Master Mendelian genetics — monohybrid, dihybrid, test cross. Do at least 20 Punnett squares until they become automatic. Know the standard ratios (3:1, 9:3:3:1, 1:2:1) and what conditions produce each.

Week 2: Deviations from Mendelism + sex-linked inheritance. ABO blood group genetics is a must — practice parent genotype → offspring possibilities problems. For sex-linkage, always use $X$ -superscript notation and make the Punnett square.

Week 3: Chromosomal disorders and pedigree analysis. Memorise the karyotype for each disorder (5 major ones). For pedigrees, learn to identify autosomal dominant, autosomal recessive, and X-linked recessive patterns from the family tree.

In pedigree analysis for NEET: if two unaffected parents have an affected child, the trait is recessive. If affected fathers never pass the trait to sons (only to daughters as carriers), it’s X-linked recessive. These two rules solve 80% of pedigree questions.

Common Traps

Trap 1 — 3:1 is the phenotypic ratio, 1:2:1 is the genotypic ratio for a monohybrid cross. NEET may ask for one when you’re thinking of the other. Always check whether the question asks for phenotypic or genotypic ratio.

Trap 2 — In incomplete dominance, $F_2$ phenotypic ratio = genotypic ratio = 1:2:1. Because all three genotypes produce different phenotypes, there’s no “masking” of heterozygotes. The 3:1 ratio doesn’t apply here.

Trap 3 — Colour blindness and haemophilia are X-linked recessive, not autosomal. A carrier woman ( $X^C X^c$ ) is phenotypically normal but can pass the allele to sons. An affected son got the allele from his mother, not his father.

Trap 4 — Down syndrome is trisomy 21, not monosomy. It results from non-disjunction during meiosis, producing a gamete with an extra chromosome 21. The child has 47 chromosomes total (not 45).